Your Search Engine Optmisation (SEO) Keywords are the Keywords and Phrases in your web content that make it possible for people to find your site via search engines such as Google, Bing, Yahoo, Baidu and Yandex.
A search engine crawls your site using spider algorithms to associate Keywords with it.
A website that is well optimized for search engine spiders "speaks the same language" as its potential visitor base with keywords for SEO that help connect searchers to your site.