A boolean, or bool value, is a very simple type of data. It can be either true or false. (See BoolValue) That's it. In Lua, everything that has a value is treated as true when converted to a boolean, unless it is nil or false.
A boolean is a bit, a bit can be either 1 or 0, and is the most important "component" in a computer. In most programming languages, 0 is treated as false. However, in Lua, 0 is not treated as false, it is instead treated as true.