Someone who reads your manuscript with no prior knowledge of the story, characters, or your intentions as the author.
A cold reader is someone who approaches your manuscript without any context. They haven't heard you talk about the plot, they don't know your characters, and they haven't read earlier drafts. This fresh perspective is invaluable because they experience your story exactly the way a future reader in a bookstore will. If something's confusing or a character's motivation doesn't land, a cold reader will catch it.
You can never truly read your own work cold. You know too much about what you meant, so you fill in gaps that exist on the page without realizing it. A cold reader exposes every place where the story in your head doesn't match the story on the page. Their confusion is diagnostic gold.
Your mom saying "it's great, honey" isn't useful feedback. Find readers who will be honest, even when it's uncomfortable. Writing groups and critique partners are better than close family.
The whole point is a blank-slate reaction. If you explain your themes or intentions beforehand, you've contaminated the read. Hand over the manuscript and say nothing.
One person's reaction is an opinion. Three people saying the same chapter is confusing is a pattern. Aim for at least two or three cold readers to separate personal taste from genuine issues.
Don't send a rough first draft to cold readers. Self-edit first so they can focus on real story problems instead of getting distracted by typos and unfinished scenes.
Write a one-page description of what you want to learn from a cold reader, then identify three people in your life or writing community who fit the criteria: they read your genre, they'll be honest, and they haven't heard you talk about this project. Draft a brief, context-free request asking them to read one chapter and note where they felt confused, bored, or pulled out of the story.