Meta keywords is a tag in the head of an HTML document that lists keywords relevant to what the page is about.
The search engines used to look to the meta keywords tag for information about the page, however, after much abuse from spammers the search engines now ignore the meta keywords tag as a signal. Many webmasters still implement a meta keywords tag on many of their pages but it provides no SEO benefit at all.
A meta keyword tag for a page about mountain bikes might look like this:
<meta name="keywords" content="mountain bikes, mountain biking, bicycles, off-road cycling, cycling">
Spammers would simply pack high-volume keywords into the tag rendering the relevancy signal useless.