The pattern represents a regular expression

[ ] denotes a range of characters within the bracketed boundary, in this case a single space.

The following * denotes "0 or more occurences".

Thus, the pattern, [ ]* denotes a pattern match for 0 or more spaces. Places either side of a comma they produce the effect of mathcing a comma surrounded by spaces and then converting the matched string to a single comma.

You may also want to check up on the use of [:space:] which essentially matches any whitespace (i.e. tabs also).