Backup is making copies of the files on your computer. If something happens to your hard drive you will sill have your information.
Backup can be done in several ways. You can use an online service, run a backup program or copy files.
There are a number of commercial online services you can use to copy backup your data to a place on the cloud. One that is promoted a lot is Carbonite. These services charge a monthly fee.
You,can use a backup program which compresses and puts your data in one large file. You must use the same program to restore your data from the backup. Windows includes a backup program in Accesories. You want to put that file on a external drive.
The third way is to just copy the files you want to backup to an external drive. For instance I copy my pictures and web page files I develop weekly to an external hard drive. You can use the copy and paste function or drag to new location.
The important part is backup regularly and know how to get data from the backup