This window is displayed when you press the Clear button in the Till Balancing screen.

The fields are as follows:
New Cash Float - this is float amount for the new till. It defaults to the current float amount. If you modify this, then an Adjustment transaction will be added to the till balance being cleared to reflect the change in float amount.
Deposit Account - choose the account into which the cleared amount is to be deposited

Press the OK button to proceed, else Cancel.