Flow through is simply that AA has agreed to fill a certain number of new hire class spots with pilots from their wholly owned regional airlines, no interviews required. At PSA that number is 6 per class currently, and those pilots are taken from the seniority list in seniority order.

Much has been written about the efficacy of said flows, but I think the bottom line is that it's a good thing to have in your back pocket for the long run. If you want to go to AA, people have been hired outside the flow, so it is possible to bypass that wait. It's also more likely to get hired at another Mainline carrier (DAL, UAL, FDX, UPS, etc) while you're waiting for your turn to flow.