Sometimes you will see people use a .(period) on the command line like an address. This isn't just a period, it means something very specific. The single . means “This directory” basically whatever directory you happen to be working at that moment. Example if you type “cd .” it won't produce an error because you just “changed directories” to the directory you're currently in! (How useful)

Also you may see two dots used .. this means it refers to one directory lower in the file tree. Example if you're in ”/home/user/My Documents” and you type “cd ..” you will then move to ”/home/user”