If

If[condition, t, f] gives t if condition evaluates to True, and f if it evaluates to False.

If[condition, t, f, u] gives u if condition evaluates to neither True nor False.

Examples

Simple conditional:

If[2 > 1, "yes", "no"]
(* "yes" *)

Conditional in a function:

f[x_] := If[x >= 0, Sqrt[x], "undefined"]
f[4]
(* 2 *)

Please visit the official Wolfram Language Reference for more details.