Steem power is what determines your vote value. So basically if you add more steem power, it can help you to get votes that are worth more.

So for instance, someone with a steem power of 3000, voting power of 100% and a voting weight of 100% can upvote a post to add $0.511 to its value. In contrast, someone with 300 steem power with a voting power of 100% and voting weight of 100% can only add $0.051 to a post's value.

A great resource for this is https://steemnow.com/upvotecalc.html