just like real world stocks, a vest represents a percentage ownership of the steem-iverse.
The blockchain explorer site Steemd shows how much Vested STEEM each steemian has, just visit www.steemd.com/@steemitusername (replace with the username you want to check). It's different from the Steemit Wallet, that shows the amount of STEEM it would take to purchase the amount of vests the user have at that time. This means a vest is a small fraction of a STEEM, so tiny that usually is represented with the unit Mv (Megavest or 1 Million Vests).