On the Define your search conditions page, type a keyword query and add conditions to the search query if necessary. If you leave the keyword box empty, all content located in the specified content locations is included in the search results. Alternatively, you can use more complex queries that use a Boolean operator, such as AND, OR, NOT, and NEAR. You can also use a keyword phrase (surrounded by parentheses) in a row. For more information about the keyword list and search statistics, see Get keyword statistics for searches.”]
Source: https://docs.microsoft.com/en-us/microsoft-365/compliance/content-search?view=o365-worldwide