As you know, the part of the knife you hold onto is the handle. There are, generally, two types of handles.
The first is when the handle material is also the liner material.
The liners are the metal parts that "line" the area that holds the knife (and make the channel that holds the blade when the knife is folder.
The second is when a different handle material (wood, micarta etc.).
For most purposes, we are speaking of the second type.
When making a liner lock, one of the liner (there are two; left & right) is bent so that it stays "sprung" in a place that blocks the blade from closing. When the knife is closed, it places pressure on the liner, so when the blade is open, the bottom of the blade clears the liner, allowing it to spring out, blocking the blade.
The pressure needed to push the liner out of the way (to close the blade) is done by the thumb.
The liner lock is one of those things that is hard to verbalize, however, if you saw it in action you would understand it immediately. I hope I've done a good enough job explaining it.
