There are tons of HTML primers online. Here's one. Here's another. Both of them have at least a rudimentary discussion of the META tag, but here's my explanation:

The META tag is a tag that goes in the header of your document that gives information to the browser about the content of your page. Most of the time, if you are just viewing webpages you won't be able to tell the difference between ones that have META tags and ones that don't, but they provide very useful information to search engines and other indexing programs. One way to use a META tag is as follows:

< META HTTP-EQUIV=type CONTENT=content >

Here, type is usually either refresh, keywords, or reply-to.
If it is refresh, content should be a positive integer. This tells the browser how often, in seconds, you want it to refresh the page. This is useful if you have a web-cam, for example, since you'll want the page to be refreshed every few seconds so that the picture changes.
If it is keywords, then content should be a string delimited by double-quotes of words or short phrases separated by commas, which briefly describe the contents of your site. Search engines use this information to automatically index pages in their database for easy retrieval.
If it is reply-to, then content should be the e-mail address, delimited by double-quotes, of the person that you should contact with questions about the website. This is not a very common tag. Usually there is just a link to the webmaster's e-mail on the home page.

Another way to use a META tag is:

< META NAME=name CONTENT=content >

In this format, name is usually author, description, or copyright. In each of these cases content should be a string delimited by double-quotes containing the name of the author, a description of the website, and the copyright of the website respectively.

There may be other uses for the META tag but these are the most important and the only ones I can think of right now.