The relevant link and eligibility criteria have already been given above.

It's essentially for First Class on AA three class products. In theory BA First passengers on their outbound sectors can use it, but there's usually an agent hovering around the front and is specifically looking for flights that are relevant for that airport - so at T8 they'll mainly be looking for the 3 class LAX services at that stage in the day. I've seen people turned away who looked eligible to me. Those Prems and GGLers who get Concierge Key treatment are also shown through here.

All it is, however, is a dedicated check in desk area by security, and someone channelling the checked passengers towards the front of security. If you are TSA PreCheck and HBO then the advantage is marginal, and sometimes the TSA are unimpressed by AA trying to push people to the front. What's promised for T5 will be rather better, and I would argue that the existing North Fast Track at T5 is miles better than the Flagship entrance at T8, MIA or LAX.