That's the funds that are not available for spending. It's either unconfirmed transactions, mined coins, or time-locked transfers. It may also contain the change: when you send money to a recipient, Bytecoin actually takes more than it requires for the transfer, makes the transaction, and returns spare coins back to you. So locked amount may temporarily increase after you've sent the money.

The funds are getting unlocked fast. Ordinary transactions require 2-5 minutes, while newly mined coins take about 20 minutes to get unlocked.