The search box that you can see in the header at the top of the page is just a keyword search. It will search for your keyword in all titles, usernames, summaries, tags, and author's notes. This makes it less useful for finding a specific fanwork but possibly useful for finding a tag that you want to read in.
You can make this search box more specific by telling it what part of the Archive you want it to look for your keyword in. Let's use the word "supernatural" as our keyword.
If you just put supernatural in the search box, your results will include titles with the word supernatural in them, usernames that include supernatural in them, summaries that use the word supernatural, tags with the word supernatural in them, and author's notes with the word supernatural.
If you want a story with the title supernatural, type title: supernatural
If you want an author with supernatural in their username, type creator: supernatural
If you want a work with the word supernatural in the summary, type summary: supernatural
If you want a work with the tag supernatural (this can include the fandom or relationship or character or additional tags), type tag: supernatural
One other way to make that search box more useful is to use quotation marks. If you're looking for a tag or a title or a phrase in a summary that is more than one word, put that phrase in quotation marks to indicate that you want those words, together, in that order.
For example, you could search for an additional tag like "fluff and angst" and get those words in that order.
This search box is most likely not the most useful way for you to find what you're looking for, but it can definitely be helpful in situations like finding a fic based on a phrase you remember from the summary or like getting to a tag that you want to tap into and then filter.