SetDelayed

lhs := rhs assigns rhs to be the delayed value of lhs. The rhs is maintained unevaluated and evaluated afresh each time lhs appears.

Examples

Define a delayed rule:

f[x_] := x^2
f[3]
(* 9 *)

Compare with immediate assignment:

rand := RandomReal[]
rand (* different each time *)
rand (* different each time *)

Please visit the official Wolfram Language Reference for more details.