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, Johann Stein appears to have found some clues to the generator's locations in ruins found in a museum in Cairo. Serious Sam manages to find photographs of the clues and Stein's remains, 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 plows 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, he heads out to the first generator location; Karnak.
Hellfire flies Sam to Karnak, where he activates the first generator that powers the Time-Lock. He then travels to Luxor and activates the generator there as well. When both generators are activated, Quinn tells Sam that he can go back to headquarters, as his mission is complete.
While traveling 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 suppoed ot 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 was 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.