What is "Google Suggest" or "Autocomplete"?

Google Suggest is the name of Google's auto-complete function. If a user enters a letter or a word in Google's search field, they are automatically shown associated terms in a dropdown menu. These suggestions are generated based on the most frequently searched terms.

Google Suggest suggests you completions of the search string you are typing, based on other people's search queries. If some suggestion often appears, this is an indicator for high popularity (resp. search volume) of the corresponding search term.