While using Burpsuite, you have to use a proxy on your browser. A proxy is an intermediate point for the traffic. So when you make a request from the browser, the request is sent to burpsuite first. Burpsuite captures this request/traffic and allows the user to modify the data. After that, the request is forwarded to the webserver. Burpsuite acts as an intermediate that captures requests and allow modification. And that's how it modify browser requests.