What is 20 off of 80?

Could you please explain what exactly does it mean to calculate "20 off of 80"? I'm trying to understand how to apply this mathematical concept in the context of financial transactions or perhaps discounts on products. Essentially, I'm looking for a step-by-step explanation on how to determine the resulting amount when you subtract 20 from 80. Additionally, is there a particular terminology associated with this kind of calculation that I should be aware of in the realm of cryptocurrency or finance?