It is called to a word or a group of words that a user writes in a search engine to look for a topic. When you create a web page, you can select a keyword for it, and later search engines will use that phrase to know if they have to show your page to the user on the search result page.
Keyword in SEO
It is extremely vital to select the right word or phrase for each web page you create to be able to rank on search engines. There are different ways to do so technically depending on how your website is built.
For example, if you are using WordPress, there are different plugins that give you the ability to write one or more words and phrases for each web page you create.
Failing in selecting the right word for each page will kill the SEO of your website.