BlankNullSequence

___ (three underscores) or BlankNullSequence[] is a pattern that can stand for any sequence of zero or more expressions.

Examples

Match zero or more arguments:

f[x___] := Length[{x}]
f[]
(* 0 *)

f[a, b, c]
(* 3 *)

Extract first element or default:

{1, 2, 3} /. {x_, ___} :> x
(* 1 *)

Please visit the official Wolfram Language Reference for more details.