The site search looks for content that contains all of the exact, case-insensitive keywords that you specify, that have at least 3 letters.
Use speech marks to search for an exact phrase. For example: "adult onset stammering".
To be less specific, use OR (upper case) between the keywords. For example: help OR advice OR support to find content with at least one of those three words.
You can exclude additional keywords by adding a minus character before them. For example: meeting -Zoom to search for content containing 'meeting' but not 'Zoom'.