Apply

f@@expr or Apply[f,expr] replaces the head of expr by f. 

f@@@expr or Apply[f,expr,{1}] replaces heads at level 1 of expr by f.

Apply[f,expr,levelspec] replaces heads in parts of expr specified by levelspec. 

Apply[f] represents an operator form of Apply that can be applied to an expression.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.