The Top 10 drop down filter boxes will show the top ten most popular tags for each category. To filter by any other tags, you need to use the 'Other Tags' field below them.
If the tag that interests you isn't in the Top 10, scroll down to the 'Other Tags' field, and start typing the tag you need -- all types of tags are valid here, and you can add as many tags as you want. The autocomplete will help you find the common (canonical) version of the tag. Use the forms that show up in the auto-complete (common or canonical forms) in order to take full advantage of the wrangled tagging structure, and find all works tagged with tags with synonymous meanings and subtags.
You can enter tags that aren't in the auto-complete, but in that case the filters will do a simple text match and may bring up unexpected results. The "Search within results" field will text-match more accurately, especially in the case of relationship tags and other tags with '/' or other non-text characters.
Choosing any tag in the Top 10 or writing in a tag in the 'Other Tags' field will do an 'AND' search with all the tags you select. This means that if you're filtering the works tagged with the F/F category, select the Teen and Up Audiences rating, the common (canonical) Romance tag in the Additional Tags dropdown, and type or select the common (canonical) tag Drama in the Other Tags field, only works tagged with all these tags will appear.
To exclude things or get results with Tag A OR Tag B, use the 'Search within results' field.
To look up tags and see which ones are common (canonical) use the Tag Search.
To read the guidelines the Tag Wranglers use to mark tags common (canonical), among other things, or to better understand the AO3-specific vocabulary for tags, read the Wrangling Guidelines.