Aliases allow you to map any URLs you create to any available resource of your blog.

Alias URL
Alias URL is the URL you create, without your blog URL. An alias URL named "aboutme" will be caught when someone call http://yourblog/aboutme page.
Alias destination
Alias destination is the resource your alias is pointing to. If you create an alias called "aboutme" and want it to serve your static page "aboutme", alias destination will be "pages/aboutme".
Regular expressions
You can use regular expression by wrapping alias URL with "/"; ie. "/^word\/(.+)?$/". You can use captures in destination; ie. "tag/$1".