Keywords are ideas and topics that define what your content is about.
In terms of SEO, they're the words and phrases that searchers enter into search engines, also called "search queries." If you boil everything on your page — all the images, video, copy, etc. — down to simple words and phrases, those are your primary keywords.
As a website owner and content creator, you want the keywords on your page to be relevant to what people are searching for so they have a better chance of finding your content among the results.