split-string is a compiled Lisp function in ‘subr.el’.
(split-string STRING &optional SEPARATORS OMIT-NULLS TRIM)
Split STRING into substrings bounded by matches for SEPARATORS.
The beginning and end of STRING, and each match for SEPARATORS, are splitting points. The substrings matching SEPARATORS are removed, and the substrings between the splitting points are collected as a list, which is returned.