Keywords are the building blocks for developing your topic.
Think of them as hashtags--keywords should be basic, short words or phrases that succinctly describe your topic. You'll use these keywords to search for research material, help you create your research question and thesis, and narrow or widen your topic.
When coming up with all the keywords you'll need, you'll need to come up with alternatives; not everyone uses the same words when describing a topic. At minimum, you’ll need your "main" keywords, a broader version, at least one equivalent or synonym, and a more narrow version.
Your "main" keywords are the words in your topic that define your entire topic. Take these words away and there isn't a topic anymore.
For instance, if my initial topic idea revolves around college students' level of stress and how it relates to exercise, I need the keywords "college students," "stress," and "exercise," without which there would be no topic.