Repeater allows the tester to submit the same request recursively by making modifications to it and checking how the server responds. Let's say the tester is testing for an SQL injection or command injection flaw on one parameter of a particular request. The tester can capture the request in Proxy and send it to Repeater, manipulate the parameter and send it to the server check response, manipulate it again, and check the response. It's like a manual debugger. Check the following screenshot for a clear understanding of the first request, which is a simple login request:

It responds with OK. However, if I change the value of the username parameter to a single quote ', then the application throws an SQL error, as shown in the following screenshot:

So as you can see, Repeater allows you to recursively modify a particular request, send it to the server, and then analyze the response.