I will explain each line.
The first three lines prevent people from viewing the contents of your .htaccess file. (Security)
The 4th line turns the rewrite engine to on. This must be turned on in order to do the rewrites.
The 5th line turns off autmatic index creation on directories without an index file. (Security)
Now on the the actual rewrites.
The 6th line uses only one variable. So if I call www.site.com/profile10.html the server extract the value between "profile" and the ".html" and makes $1 the value. So now member.php?ppaction=profile&uid=$1 becomes member.php?ppaction=profile&uid=10
The 7th line uses two variables. I call www.site.com/album23-user4.html the server extracts the value between "album" and "-user" making this value variable $1. The next portion of the url is between "-user" and ".html" taking this value variable $2.
So now album www.site.com/album23-user4.html becomes www.site.com/showgallery.php?cat=23&ppuser=4
You can take these examples and make .html pages with as many variables as you wish.
Hope this helps.