RuleDelayed

lhs :> rhs represents a rule that transforms lhs to rhs, evaluating rhs only after the rule is used.

Examples

Use RuleDelayed to delay evaluation:

{x, x, x} /. x :> RandomReal[]
(* {0.234, 0.891, 0.456} *)

Compare with Rule (immediate evaluation):

{x, x, x} /. x -> RandomReal[]
(* {0.789, 0.789, 0.789} *)

Please visit the official Wolfram Language Reference for more details.