preg
means Pcre REGexp", which is kind of redundant, since the "PCRE" means "Perl Compatible Regexp".
Regexps are a nightmare to the beginner. I still don’t fully understand them and I’ve been working with them for years.
Basically the example you have there, broken down is:
"/[\s,]+/"
/ = start or end of pattern string
[ ... ] = grouping of characters
+ = one or more of the preceeding character or group
\s = Any whitespace character (space, tab).
, = the literal comma character
So you have a search pattern that is "split on any part of the string that is at least one whitespace character and/or one or more commas".
Other common characters are:
. = any single character
* = any number of the preceeding character or group
^ (at start of pattern) = The start of the string
$ (at end of pattern) = The end of the string
^ (inside [...]) = "NOT" the following character
For PHP there is good information in the official documentation.