Here's the short version.

A normal battery is a sealed cell. It has a positive and negative electrode, with an electrolyte between them. Usually many layers of this. When you charge it, a chemical change happens. When you discharge it, that chemical change is undone.

A redox flow battery uses fixed electrodes, but a liquid electrolyte that can be pumped and stored. This means you can increase overall storage capacity simply by adding more electrolyte tanks, without needing more electrodes. Think of it like a generator with a bigger gas tank.

The whole vanadium thing is just one of the metals used in the battery. There's a few kind of redox flow batteries using different chemistries