TSA employees have always been allowed to join a union and let the union represent them in workplace issues. TSA employees do not have collective bargaining rights and this vote will not change that.
There is currently two unions fighting to represent TSA employees. This vote will simply settle that fight once and for all. It will determine which of the two will be the sole representative of TSA employees.
The unions basically want the same thing and have the same ideas and goals. The difference between the two is how they go about achieving their goals. One has a reputation of treating agencies and management as the enemy and the other treats the relationship as a partnership and tries to work with agencies to reach an agreement that satisfies both sides and achieves labor harmony, but will sue as a last resort.
From what I've seen, the union that doesn't act like a junior teamster from the Hoffa days seems to be the union that does better for it's members when it's time to negotiate contracts.