Burp Repeater allows to take the request captured in the proxy, edit it, and send the same request repeatedly. We can also craft and send requests by hand.
Whilst we can craft requests by hand, it would be much more common to simply capture a request in the Proxy, then send that through to Repeater for editing/resending.
With a request captured in the proxy, we can send to repeater either by Right Click -> Send to Repeater (or ⌃ + ⌘ + R).
⌃ + ⌘ + R
Repeater offers us various ways to present the responses to our requests, from hex all the way up to a fully rendered version of the page.
There's also Show non-printable characters button (\n), allowing to display characters that usually wouldn't show up in the Pretty or Raw views.
The Inspector is a way to get a prettified breakdown of the requests and responses, as well as for experimenting to see how changes made using the higher-level Inspector affect the equivalent raw versions.