Meta in any language and HTML is no exception is the raw data that defines a page. What you are saying to the computer or reader is "I am a Letter", "This letter is about me moving house", and in HTMLs case these are some key words "united,kingdom,midlands,London,moving,house".
They are set in the header or rather <head> </head> tags.
<meta http-equiv="Content-type" content="text/html; charset=utf-8" /> - Tells the browser that this is written in HTML using that character set.
<meta name="description" content=" This letter is about me moving house"/> - Description Meta
<meta name="keyword" content="united,kingdom,midlands,London,moving,house"/> - Keyword Meta
There are more Meta tags than this of course.
The reason why we use them is because of Spiders from Google can pick up on the Meta tags. So your title or rather <title>I am a Letter</title> becomes the title in the search engine and then the description is from the meta along with the keywords.
Read more: [wiki.answers.com