The meta keywords tag is a piece of code placed in the head element of an HTML document of a page that allows you to list the keywords you'd like to target.
A typical meta keywords tag:
A long time ago when the internet and SEO were just in their beginning, this tag factored heavily into ranking decisions for search engines. Today, most search engines such as Google do not use the meta keywords tag as a ranking factor, though there are some search engines that still use them.
If you're optimizing for a search engine that uses meta keywords in their ranking algorithm (Baidu), it's definitely a good idea to include it on each page you want to be reached via organic traffic.
For other search engines, it remains a controversial topic on whether or not to include the meta keywords tag on your site. There are two different schools of thought here:
No: Since search engines don't use this tag for rankings, some say it's a waste of time to include them on your site. Furthermore, since the source of each URL is publicly available, competitors can easily see which keywords you're targeting and use this information to rank against you. Finally, some search engines like Bing only use this tag to detect webspam. So you want to be sure you're not over-optimizing this tag by adding too many keywords in it.
Yes: Others say that while it's true search engines don't include meta keywords as a ranking factor, most do look at this information, so there could be other benefits to including meta keywords that we may not know about. They feel that it's usually pretty easy for a competitor to successfully guess what your targeted keywords are just by looking at your content, so including them does not really give away any secret information, though it may just make it slightly easier.
Guidelines for Writing Meta Keywords Tags
Only include the top 3-4 keywords you're targeting on the page. (Best practices say not to target more than 3 keywords per URL anyway).
List each keyword (or keyword phrase) descending by importance, comma-separated.
The meta keywords tag should be no more than 100 characters long.
Avoid keyword stuffing and duplicating the same keyword.
Each meta keywords tag should be unique. No URLs should have the same meta keywords value.