While Temple Run is free, you can buy things from the store either with real money or by using Temple Run Coins that you collect along the way. You will find them pretty much throughout all of the levels you play and they are color coded according to their value:
Blue are the best worth 3 Coins
Red are pretty good worth 2 Coins
Yellow are the basic coins worth just 1 coin.
If you are not sure what to look out for keep an eye out for diamond shaped icons in the colors mentioned above. The blue and red only appear after certain distances:
The red coins (worth 2 coins) will only appear after 1,000 meters.
The blue coins (worth 3 coins) will only appear after 2,000 meters.