#1
What do brackets in a tab mean? some times I see ones like
(12) and sometimes it's like 12b (12) what do they mean?
#2
...by the way you mean parentheses and not brackets....
HERE IS WHAT I FOUND AND ALWAYS THOUGHT MYSELF....

Optional/Ghost notes.
Either pick them lightly, or just don't pick them at all, it'll still sound

It usually means first of all that you don't intentionally try to play the note. The note will still sound, either because it will vibrate sympathetically from another string. Or that it might or can sound because it's next to another string being played and might be brushed (or sound sympathetically) as the other note is played. Some tabbers put it in as an optional note. It can be played or not played. That's usually because on the original recording you can definately hear it but it can't be determined whether it's actually played or just sympathetic. Different tabbers have slightly different ways of showing those ghost-notes, etc and so it can mean different things, especially in different styles of music. Just be aware of what it might mean and treat it the way that makes the tune you're working on sound best.