well, you can read about it in the official apache website here:
http://httpd.apache.org/docs/1.3/mod/mod_rewrite.html

but for you, whats important to know is that with mod rewrite you can rewrite a url into a different url.

for example, lets say you have a blog and the url to access one of your blog pages is:

http://www.myblog.com/blog.php?blog_id=1945

as you can see from the above example, when you visit that page, it should open blog_id 1945 from your database,

but how about if you can have it the url like this instead:

http://www.myblog.com/1945.html

as you can see, the url is much shorter and it will display the same content

thats what mod rewrite can do for you. i believe wordpress calls these "permalinks"