Quoting the white paper:

In order to compensate for the ever increasing precision, the STEEM network performs a 10:1 “reverse split” every 32,000,000 blocks (about 3 years). At this point in time all balances of STEEM are divided by 10 and all prices are multiplied by 10. Cryptocurrency exchanges will have to suspend trading around this time and update the account balances and price history to reflect the “reverse split” before resuming trading.

The "ever increasing precision" thing means that as new STEEM is minted, the total number of tokens will eventually exceed the maximum number a modern 64-bit computer can represent. Without the split, token supply would increase 1000 times in 10 years.