A flow is a path through your product that has to keep working: sign up, add to basket, check out. Recording one gives you something to check a change against.
Recording happens in the browser panel. You walk the path yourself and the panel captures the steps, the network activity, and what the page looked like at each point.
What it is for. When someone — you, a seller, or an agent — changes something nearby, the flow is the thing you replay. It is far cheaper than finding out from a customer that checkout broke on Tuesday.
What it is not. A flow is not a test suite and does not replace one. It records what a real path looked like when it worked, which is exactly the context missing from a bug report that says "checkout is broken".