It means it is "not-" whatever is in front of it.
If it's attached directly to a verb stem, then it negates the verb, but you an also place it after another suffix to negate that suffix.
For example, Qal "swims" versus Qalbe' "does not swim", but then you can also say QallaH "can swim" versus QallaHbe' "cannot swim" (the -be' "not" negates the -laH "can"). If you switched the order, Qalbe'laH would mean that it's possible for you not to swim, i.e. the -be' negates the Qal (doesn't swim) and then -laH shows that you can "not swim".