Member dollars are earned by writing reviews. You use them to promote your writing and get more feedback.
Contests that do not have a cash prize will reward a prize pool of member dollars. If a contest has a member dollar entry fee that entry fee goes into the contest prize pool that the winner will receive.
You can control your position on the listings. The more member dollars you own, the more you can promote your writing. The higher on the listing your writing is the more feedback you can expect.