Let's say you're interested in finding articles about space tourism. You type your keywords into your favorite database, and you get a lot of results -- almost 700,000!
Some of these sources are about space tourism, but many others are not. They have the words "tourism" and "space" in them, but not necessarily together as "space tourism".
So, for example, this result is about tourism spaces. Definitely not about space tourism.
In most databases you can eliminate irrelevant results like these with a feature called phrase searching. By putting quotation marks around the phrase you want to search, you're asking the database to search for that exact phrase as you have typed it.
Searching for "space tourism" again, this time with quotation marks, you only get a little over 20,000 results. That's a lot fewer than your first search, but they are more relevant, because all of these articles mention "space tourism" as a phrase.
If you have any questions about phrase searching or about any other search strategies, ask a librarian for help!