Implementation of stdin and stdout

Underscore is just as valid a variable name as x, for example.

That is, from the interpreter's point of view, the expressions for _ in range(5) and for x in range(5) are no different and will execute identically.

It's just a tradition to use underscore to indicate that this variable is not used anywhere else. That is, we are using some syntactic construct that requires creating a new variable, but we don't need this variable itself, but rather some other effects from this syntactic construct.

The expression for _ in range(5) is usually used simply to repeat something 5 times. On each iteration, the variable _ will sequentially take the values 0 through 4.