In general, large search engines such as Google, Yahoo!, and Bing, do not take your keywords in account. At least, so they say. Yet, most SEO expert will tell you that including keywords on your pages is a very important idea. The main reason why is that those words are viewed as words being highlighted.
To make an analogy: by adding keywords in the Snap! Keyword Meta Data you make all those keywords bold face.
Keywords are written one after another and separated by commas.
For example, this very page can use the following keywords: keywords, meta data, Snap!, SEO
Note that in most cases keywords are entered in lowercase unless they represent an invariant name such as Snap! in our example, or an abbreviation usually written in uppercase, such as SEO in our example.
Keywords can be multiple words, as shown in our example: meta data. It should not be a sentence, however.
Snap! also offers the option to enter tags on a page. These tags can then be added to the Keyword Meta Data field. When used along the Automatic Add Terms feature, it is very powerful since the most used words automatically become your page keywords.
Thus, this meta tag can either be used manually or as the automatic keywords gatherer.