The Backpack gives you the opportunity to free up cells on the merge field by moving items into it. To move items into the Backpack, simply drag them there. To put them back on the field, tap an item in the Backpack and it will return to any empty cell.

Once you run out of room in the Backpack, you can buy an extra pocket for banknotes.

Please, notice: if you put a Generator into the Backpack, it doesn't regenerate. The timers don't decrease there either. Also, the item that lies in the Backpack won't be counted in the task -  it should be on the merge field to be counted.