Convergence tests are used to determine when it's safe to end a calculation. They are used when someone is doing a calculation that involves adding up lots of small numbers or doing a calculation many times in a row (called iterations).
For example, if you wanted to know how many quarters you would get if you combined 2 nickels, 3 dimes, and 4 quarters, you could use a convergence test to help you determine the answer. In this example, the convergence test would tell you when to stop adding. It might say something like 'add numbers until the difference between two consecutive additions is less than 0.01', which means you would add until two additions in a row were the same to the nearest 0.01. Once the difference between two additions was 0.01 or less, you would have your answer.