AND
The AND operator tells the database that the terms on either side of the operator are both important and need to appear in a result for it to be relevant to you. AND narrows your results because your search criteria is higher.
Think about this search: Cat AND Meme
This search will only return results that include both the term cat and the term meme. The left circle below represents the results from the search for cat. The right circle below represents the results for the search for meme. The shaded area where they overlap shows you results for Cat AND Meme. This is a smaller number of results, but if what we're looking for is information on cat memes, these results are much more likely to be relevant.
Use AND to connect keywords that describe different main ideas.
Many databases assume an AND operator when you search for more than one word. If you're just doing a search for Cat Memes, you don't have to think about boolean operators too much, but it's important to know what this operator is doing for your searches.