Please don't use things like "help me" in your subject. It's too vague and won't attract the attention you need. Read the link in my signature for details.
In any case, the mount command is what connects a drive or partition (hard disk, floppy, CD, USB stick, network share) to your system so you can access it. In this cse, it mount the device named /dev/sda1 at the location /mnt/usb, with automatic detection of the filesystem type.
There's plenty of information on how mounting works in Linux. Try google: "linux mount". Also read the mount man page: "man mount".