Vests are like shares in Steem Power. When you power up, your vests increase, when your vests increase it means you have more influence on Steemit. The higher your vests the more your vests are worth. This article has more details than I can give in a single comment, check it out https://steemit.com/interest/@sigmajin/understanding-the-steem-economic-system-vests-sbd-steem-dilution-interest-and-all-those-crazy-things