Level

Level[expr, levelspec] gives a list of all subexpressions of expr on the specified levels.

Examples

Get elements at level 1:

Level[f[g[a, b], h[c]], {1}]
(* {g[a, b], h[c]} *)

All levels:

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

Please visit the official Wolfram Language Reference for more details.