The 300 credits can be applied to your websites you are advertising..
When you log in, click on "My Sites" on the left side menu. You will see this page below..
You can see, I added 3 sites.. Each one of these is a different advertisement rotating in their system.. The "300 credits" you mentioned can be applied to your ads.. Each credit is a "view" of your ad.. So, you have right now 300 credits that can be applied to make 300 views of your ad.. And, everyday, you will get more credits, which can be applied over and over to your ads..