It takes a string (char*) and returns a pointer (char *) to the position to break out the portion of the first string at a position with a character from the charset string.
I think that this nomenclature goes back to SNOBOL pattern matching, which dates back to 1962.
There are these two complementary pattern matching operators in SNOBOL:
SPAN matches a string containing any characters in its argument list, BREAK matches a string containing any characters not in its argument list. I think the implied mnemonic is to SPAN a substring containing a character set or BREAK (stop matching) on a substring containing anything not in the character set.