When you sit at a poker table, you have a set amount of time to act, depending on the game you're playing:
If you have a difficult decision to make, you can request additional time to think about your move. This extra time is known as the time bank.
In tournaments, the time bank available varies according to tournament type. It ranges from 90 seconds for a standard multi-table event to 10 seconds for a Hyper Turbo Sit & Go. You can find this information by opening the tournament lobby and selecting the 'Structure' tab.
For cash games, the time bank on each table is 15 seconds pre-flop and 15 seconds post-flop. This will then increase by 5 seconds for every 10 hands you're dealt at that table. The pre-flop and post-flop time banks are independent of each other.
Whenever your normal time to act is about to finish, the TIME bank button will appear, and you can choose to activate your time bank:
Keep in mind that the time bank only allows for more thinking time, it's not a protection against disconnections.