Storage
When you save a program or file, your computer copies onto storage. Storage is "persistent", meaning that even if your computer loses power (because it was restarted, turned off, or maybe lost power in a thunderstorm), your files on disk will still be there when you turn it back on.
Sometimes people save files in the cloud instead of on their computer. Cloud storage is also called "remote storage" (because it's far away), and contrasted with "local storage" which is on whichever computer you're using.
There's also "removable storage" or "portable storage" which includes USB drives and external hard drives. These are convenient ways to add more storage to your computer without opening it up and installing a new drive yourself.
These days, computers come with terabytes (i.e. thousands of gigabytes) worth of storage. This means we can store a lot more files than computers used to.
Unlike memory, your files take up storage space even when they're not open. So, closing a file doesn't free up storage space. You have to delete the file to free up storage space. Programs like Disk Inventory X let you visualize which files are taking up the most space on your computer, so you can delete unimportant files and get that space back.
Also unlike memory, adding more storage doesn't make your computer any faster, or let you use more programs simultaneously. It just increases the number of files you can store.