"Decertification" of the NBAPA would mean the union would no longer have the authority to negotiate on behalf of the players -- essentially making every NBA player an independent contractor. Such a move would give the players some flexibility; they could challenge the salary cap as an anti-competitive measure under anti-trust law, for example. But they'd also be giving up any collectively-bargained benefits (pensions, minimum salaries, etc.) It's an option for the players, but it's probably a last resort.
Sports law expert Michael McCann explained decertification in more detail in an interview with Sports Illustrated.
