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 millennia later, Earth was attacked by Mental. Like the Sirians, the humans 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 refused to hand the data to the HQ, so Alpha Team was send in to rescue the professor. Sam, the only surviving member of the Alpha Team decided to rescue the professor alone, but was too late when he discover Stein's dead remains. However, Sam manages to find data 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 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 drops Sam in Karnak, he goes to activate 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 informs Sam that he can return to headquarters, as his mission is complete and Charlie Team is ready to deploy. After Sam destroys an Alcor Class Warship that is a threat for the helicopter while landing, Wilson arrives and extracts him.
Unfortunately, Quinn informs Sam that the Time-Lock failed to come online as they expected. Both of the generators are turned on but it hasn't started the Time-Lock because there's a Safety Switch (which is like a Fail-Safe) somewhere in Philae that prevented it from activating and Wilson changes course to drop Sam in Philea to find it. After Sam fights his way through the enemy forces, he manages to activate the safety switch in the Tomb of Sethirkopshef and the Time-Lock is activated. After Sam defeats the Scrapjack Boss, Wilson arrives and extracts him.
While returning 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 stretch, 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; its 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 Hatshepsut. 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 its 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 both to explore the past and to 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.