The Time-Lock is basically a time machine. However, it is designed so that it can only teleport someone back to the date it was armed. It can accept organic and inorganic objects, but will usually strip off any large inorganic objects on someone when they use it. It appears to require an immense amount of power, as there are two generators dedicated to powering it, with one in Karnak and another one in Luxor.
It was created by an unknown species, but discovered by the Sirians while exploring the galaxy. During Mental's siege on the Sirians, they sent the Time-Lock to Earth, a planet they were just starting to explore, to prevent it from falling into Mental's hands. After the fall of Sirius, the last remaining Sirians decided to arm the Time-Lock to their time in case it was needed by the humans for whatever reason. After being armed, the Time-Lock was buried in the Temple of Hatshepsut, and it was lost for many, many generations.
A few millenia later, Earth was attacked by Mental. Like the Sirians, they were slowly defeated to the point where Mental only had to defeat Earth to complete the annihilation of humanity. The Earth Defense Force discovered the Time-Lock, and began making a last-ditch plan where they would insert a team via the Time-Lock in order to kill Mental in the past. However, the big problem is that the power generators are shut down, and their locations are unknown.
At the start of Serious Sam 3 BFE, Johann Stein and his escort, the Bravo Team managed to find some clues to the generator's locations in some ruins found in a museum in Cairo but all his escorts were killed and he refuse to hand the data to the HQ, so Alpha Team was send in to rescue the professer. Sam, the only surviving member of the Alpha Team decided to rescue the professer alone, but was too late when he discover Stein's dead remains. However, Sam manages to find datas of the clues and sent it back to HQ, then starts to leave Cairo.
Sam travels to the Great Sphinx, which apparently leads to a clue. The recovered texts state that there is something under it and that there's a way inside, but Sam takes the easy way and blows the Sphinx up, which reveals the entrance to the underneath the Sphinx, Sam finds an old Sirian base and a hologram that shows where the Time-Lock's generators are and how to re-activate them. After sending this info to HQ, Hellfire arrives to extract him and flies out to the first generator's location; Karnak.
After Hellfire dropped Sam in Karnak, he goes to activates the first generator that powers the Time-Lock. After that, he travels to Luxor and activates the second generator there as well. When both generators are activated, Quinn inform Sam that he can return to headquarters, as his mission is complete and Charlie Team is ready to deploy. After Sam destroy Alcor Class Warship that is a threat for the hellicopter from landing, Wilson arrived and extract him.
Unfortunately, Quinn inform Sam that the Time-Lock fail to come online as they expected. Both of the generator's are turned on but it doesn't started the Time-Lock because there's a Safety Switch (which kinda like Fail-Safe) somewhere in Philae that prevent it from activating and Wilson change course to drop Sam in Philea to find it. After Sam fight's his way through the enemy, he managed to activated the safety switch in the Tomb of sethirkopshef, the Time-Lock has activated. After Sam defeat the Scrapjack Boss, Wilson arrived and extracted him out of there.
While returning back to HQ, his helicopter is shot down by a Major Biomechanoid and he loses contact with HQ. After traveling through some temples, Sam finally manages to contact HQ again. However, it turns out that Mental has attacked HQ and killed Charlie Team, the group that's supposed to enter the Time-Lock, among other people.
With Charlie Team dead, Sam decides to go to the Time-Lock himself and start Charlie Team's mission. He takes control of a car in a slum, then drives it to the Time-Lock until he runs out of gas. From there, he fights his way to the Time-Lock. As he reaches the final strech, he is attacked by Ugh-Zan IV. Thanks to some creative thinking, he manages to kill Ugh-Zan IV and enter the Time-Lock right before Earth is destroyed by Mental.
Sam is then warped to Ancient Egypt via the Time-Lock, and starts his journey to defeat Mental.
There is one huge inconsistency between the Serious Sam 3: BFE and Serious Sam: The First Encounter versions of the Time-Lock; it's size. In Serious Sam 3, the Time-Lock is a large and tall machine that can safely fit multiple people in it and is buried underneath a part of the Temple of Hatsheput. In addition, it is colored gold and dark blue and the four towers surrounding the Time-Lock are designed like Egyptian human shapes.. In Serious Sam: The First Encounter, the Time-Lock is significantly smaller, is a grey color, located above ground, and it's towers are not shaped like a figure seen in Egyptian statues.
In Serious Sam: Next Encounter, Mental warns the Evil Serious Sam Clone not to use his personal Time-Lock. However, the Clone ignores his advice and hops into it, which starts the problems in Next Encounter that Serious Sam has to fix.
In Serious Sam Advance, humanity has managed to adjust the Time-Lock so that they can warp anywhere in history. They use the Time-Lock to both explore both the past and make sure that Mental doesn't tamper with the past in any way. The game starts out with some agents from Earth disappearing during a routine time traveling trip. Sam is sent back into the past via the Time-Lock to see what happened.