Can I use a dongle for my I Pad when I'm in Spain

No. You cannot use it at home or abroad.

What does this "dongle" do and why would you want to use it?

By dongle, are you referring to a USB modem? (Dongle can mean pretty much anything you plug into a port. The Camera Connection Kit, is also a dongle which works with the iPad.) No, you can't use a USB modem with an iPad. If you need to add cellular connectivity, you can get a personal hotspot device, often referred to as a MiFi, from many carriers.