What is the difference between instant unstake and non-instant unstake?

Instant unstake allows you not to wait up to 72 hours, as with "regular" unstake. 

In this case, you need to independently exchange your stTON for TON on any DEX (decentralised exchange) of the TON network. 

The transaction is instantaneous, but the exchange will charge a commission, and the exchange price may differ from the fundamental price that would be at "regular" unstake with bemo.