Responses and reactions from readers about your writing that help you see your work through someone else's eyes.
Feedback is any response you receive about your writing from someone who has read it. It ranges from a friend saying 'I couldn't put it down' to a professional editor delivering a ten-page report on your story structure. The key distinction is between feedback that just reacts ('I liked the ending') and feedback that diagnoses ('The ending works because the twist recontextualizes the opening scene'). Writers need both, but actionable feedback is what drives real improvement.
Writing in isolation for too long is like practicing piano with earplugs. You need outside perspectives to understand how your story actually lands, as opposed to how you intend it to land. The gap between those two things is where your best revisions live.
Take a scene you've been struggling with and send it to two different people: one writer friend and one non-writer friend. Give each the same three questions: Where did you get confused? Where did you get bored? What do you want to know more about? Compare their answers. The overlap is where your real problems are.