Nowadays, every online poker room features a Frequent Player Points (FPPs) a.k.a. Poker Points system. PokerStars too, has its own FPP program, though you’ll be probably amazed to see how much bigger a bang you get for your FPP here, than in any other poker room out there.
PokerStars FPPs can be used for a variety of purposes, and generating some FPPs is practically impossible to avoid, if you play real money tables. Every time you play a raked hand (real money, of course) you generate some FPPs. The amount of FPPs you generate is directly proportional to the rake you produce (there are some intricate charts on PokerStars showing you exactly how this happens).
 It is important that you keep in mind that FPPs are added to your balance at the conclusion of raked hands. If you leave the table before the end of a hand, for some reason, you won’t earn any FPPs.

FPPs can also expire. If say, you don’t play poker for a year, and thus you don’t generate any FPPs during that time, your previous FPP balance will expire. 
You will also earn FPPs in any tournament game (in which there are no raked hands per se) if it features a tourney fee.

What exactly can you use these FPPs for? For one thing, you need them to unlock/redeem your bonuses.
Next, you can use them to participate in PokerStars tournaments. Instead of real money, you can buy into tourneys with these points. 
FPPs can make your dreams come true in the sense that they’ll make it possible for you to play in exclusive satellites, for different prestigious events like the WSOP, EPT and others.

Last, but certainly not least, you can take your FPPs to the PokerStars FPP store, and use them as currency. You can buy different PokerStars paraphernalia and memorabilia, like caps, mugs, but also 500 piece clay chips-sets, and a variety of poker books.