The fields for Fandoms, Characters, Relationships, and Additional Tags suggest tags as you type. Try to use the "canonical" or common version of a tag if the autocomplete offers one. It will return all results containing this tag and synonyms of this tag. For example, picking the common relationship tag Erika Mustermann/Juan Pérez will return works tagged Juan Pérez/Erika Mustermann as well, assuming these tags have been linked by a wrangler in the background.
If a tag doesn't appear in the autocomplete menu, it doesn't necessarily mean that the tag doesn't exist on the Archive, just that it hasn't been marked as common by a tag wrangler. You can type in any word or phrase here, without surrounding quote marks. If your phrase doesn't exactly match a common tag, it will find all tags containing the words in your phrase. Typing in People Doing Things will also find works tagged Nice People Doing Things, People Doing Shady Things, and People Doing Things with Spoons. Depending on your search, results in this case will be somewhat unpredictable.
The more search terms you enter or options you pick, the more your search results will be narrowed down: Entering two fandoms will only find crossovers of those fandoms, not all works that are in one fandom or the other. Entering two characters will only find works that have both. Picking M/M and F/M will only include a work in the search results if it has both category tags.
You can read more about tagging in our Tags FAQ.