The tills represent the actual tills that the practice uses to hold money. They track not only the cash payments that you receive, but all the other types of payment types (such as cheques, credit cards, EFT, etc) and also any refunds. There will normally be one till for each location but there can be more, and multiple locations can share a till.
At suitable intervals, normally at the end of the day, you will want to check what's in the till, and take out what should be deposited in the bank. This is done using the Reporting|Till Balancing screen. Using this you can print a report of what the system thinks is in the till; you can adjust the cash amount if there have been counting errors; and you can 'clear' the till to remove everything that should go to the bank. It is normal to leave a small cash 'float' in the till so that you have money on hand for change (and cash refunds) at the start of the next day.
Note that the system allows you to balance the till without stopping/pausing the business. Hence practices that operate around the clock are supported.
So the process is print, check and if necessary adjust, then clear.
After you have cleared the till, the money can only be deposited in the bank - it is not available for any refunds. The deposit operation is done using the Reporting|Deposit screen.
The tills are setup by your administrator using Administration|Organisation|Tills.