On Steen you get rewarded for posts and oc through votes and with steem (so basically money) and steem power. The more people vote your post up the more you get rewarded. But not every vote counts the same. If a person with more steem power votes it has a bigger impact then someone with less. And that's exactly what steem power does. It gives you the power to have a bigger impact with your votes. You should read the beginner guide in which this is discribed further