Computers are made up of logic gates. These components contain a number of inputs and an output based on those inputs, both of these things are typically binary. This means that they can have one of two values, like on or off, 1 or 0, high or low, true or false, etc. In order to describe the behavior of a logic gate a truth table is typically used which states the expected output for each input configuration.

An example of a logic gate is the nand gate, which has the following truth table:

Symbol of the Nand Gate

So for example, the nand gate is true, unless both inputs are true.

Because it is one of the few gates that is a universal gate, which means that you can make any other type of gate using it. There are other such gates but this is the simplest to make.