Vests are shares of Steem Power funds. If the user does not take any action against the Steemit Platform (upvote, post, comment), the stock will remain static in quantity, just like shares in financial funds. Also, like stocks in financial funds, the underlying asset base can change as new investors enter, increase investment, but this action does not change the number of shares you hold in the fund.
How can your vests increase and decrease?
You can increase the number of your vests by creating articles that get upvote, upvote other people's articles, or Powering Up, you can also become Witness / Miner. You can Reduce the Number of Vests by Powering Down.
How do you know how many vests you have?
You can see the number of vests that you have by visiting https://steemd.com/.