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When you power up STEEM, inside the blockchain you will be assigned a certain number of "vests" based on whatever the current price of vests are. Your vests are basically your shares in the Steem Power pool of STEEM. You keep these vests unless you power down or do other things to change your vests (mentioned in the post above). As you hold your vests, the blockchain will automatically keep adding more STEEM to your account (as Steem Power). Over time, the same vest will contain more and more STEEM.

The blockchain divides things into really small increments, so the total number of vests in the system is a really huge number. For most users, even their vests is a really large number. For example, you currently have around 423,000 vests. To make the numbers easier to work with, we usually refer to the amounts as "millions of vests" or Mvests. 423,000 vests = 0.423 Mvests.

You can check out how many vests you have here (in the upper right corner): https://steemd.com/@chadcrypto

Thank you @timecliff for the info :)