It's a word that has the same or nearly the same meaning as another word.
Let's look at some examples:
female, girl, "young lady"
These are just some examples. You know what is really easy? Try typing your keyword in Google and add the word synonym. (So try it - diverse synonym... see what happens?) That's an easy way to find other words that mean the same thing.
Also, note how I continue to keep phrases together! If I have a synonym that's also a phrase, you use the quotation marks just like before.
Look at the tricky one - "Social media". There's not another word that means just the same thing, but there are LOTS of examples, like Facebook and Twitter. It might change the focus of your search a little, but it also might help you find more articles.
PRO TIP 1
If you start using synonyms and realize your search results are not on topic at all, you may have changed the meaning just enough to lead you in the wrong direction. Make a note, and try a different word!
PRO TIP 2
When you're considering synonyms, make sure you keep your topic in mind at all times. If you can exchange your possible synonym with your keyword and your topic still make sense, then great! If your topic suddenly doesn't make sense when you swap the words, then you might need to reconsider the synonyms.