Riffle
Riffle[{e1, e2, ...}, x] interleaves x between elements: {e1, x, e2, x, ...}.
Examples
Interleave elements:
Riffle[{a, b, c}, "-"]
(* {a, "-", b, "-", c} *)
(* With multiple separators *)
Riffle[{1, 2, 3}, {a, b}]
(* {1, a, 2, b, 3} *)
(* Join strings with separator *)
StringJoin[Riffle[{"a", "b", "c"}, ","]]
(* "a,b,c" *)Please visit the official Wolfram Language Reference for more details.