The meta "keywords" tag is largely ignored these days (as it was so hideously abused by people trying to capture search results for which they weren't really relevant), but peppering your title, headers, and text with a variety of relevant words and phrases is a good idea.
For example, a page about a Unitarian Universalist (UU) child dedication ceremony should include a sentence like, "Rather than holding Christian-style baptisms, most UU congregations have child dedication ceremonies," because many searchers will be looking for information on baptisms; if that word isn't anywhere on your site, search results for that term will yield no results for your site, and users will never learn about how UUs celebrate this important life event.
As you write, mix up your language a bit to capture common variations. One paragraph might include the phrase, "search engine optimization," and another might instead use, "optimizing your site for search engines."
Using a variety of key words and phrases in your writing will both improve your SEO and make your text more interesting. Check your site's analytics to see what kind of language your users are looking for.
Your most important key words should be used in your page's title, headers, and/or description (the body summary field).