Where do the phrases come from?

Google Autocomplete: the suggestions real people see as they type, taken from the suggestion endpoint itself. They are evidence of demand, not volume estimates, and the page says so rather than inventing a number.

How do you decide a phrase is a gap?

Each phrase is compared with the words in your sitemap URLs. If no page carries most of the phrase’s meaningful words it counts as a gap; a near miss is listed as partial so you can judge it yourself.

Does this tell me what my competitors rank for?

No, and it does not pretend to. That needs live search-result data, which is a different and much bigger build. This measures what people search against what you already have, which is the half you can act on today with no tool at all.

What do I do with the list?

Write the question-shaped ones first: they are the cheapest pages to win and they map cleanly onto one article each. Then link each new page from the products it supports.

What are people searching that you have no page for?

Paste a store URL. It reads what your sitemap already covers, asks Google what people actually type around your topics, and shows the phrases no page of yours targets. Real demand against your real pages, no invented volume numbers.

What this measures, and what it does not

It measures: search phrases real people type (Google Autocomplete) against the pages your sitemap lists, and it splits them by intent: an informational question needs an informational page, so a product page never counts as covering "how to wash …". It does not measure: search volume, difficulty, or what competitors rank for. Treat the list as strong candidates for your next pages, not as a strategy with numbers attached. Any tool that shows you volume for free is estimating.

For the other half of the question (why a competitor's page outranks yours, and which subtopics it covers that yours does not), that is what the content-gap check does, using live search results rather than autocomplete.

Questions

Where do the phrases come from?: Google Autocomplete: the suggestions real people see as they type, taken from the suggestion endpoint itself. They are evidence of demand, not volume estimates, and the page says so rather than inventing a number.
How do you decide a phrase is a gap?: Each phrase is compared with the words in your sitemap URLs. If no page carries most of the phrase’s meaningful words it counts as a gap; a near miss is listed as partial so you can judge it yourself.
Does this tell me what my competitors rank for?: No, and it does not pretend to. That needs live search-result data, which is a different and much bigger build. This measures what people search against what you already have, which is the half you can act on today with no tool at all.
What do I do with the list?: Write the question-shaped ones first: they are the cheapest pages to win and they map cleanly onto one article each. Then link each new page from the products it supports.