Autocomplete shows possible answers while someone types into a field. It is like a helpful shopkeeper offering likely choices after hearing the start of a request. The person can pick a suggestion or, when allowed, keep typing their own answer.
For example, a travel form might suggest city names after someone types a few letters. Suggestions can save time and help people choose a spelling the service recognizes.
Make it clear when a suggestion is only a suggestion and when the person must select an allowed option. Keep the original typed text visible so the person can check what they entered before continuing.