Splice

Splice[{e1, e2, ...}] represents an expression that will automatically be spliced into any list in which it appears as the sequence of elements ei.

Splice[{e1, e2, ...}, hpatt] represents an expression that will automatically be spliced into any expression whose head matches the pattern hpatt.

Examples

Splice elements into a list:

{1, Splice[{2, 3}], 4}
(* {1, 2, 3, 4} *)

Splice in function arguments:

f[a, Splice[{b, c}], d]
(* f[a, b, c, d] *)

Please visit the official Wolfram Language Reference for more details.