Help me understand pet insurance deductible!!!!!!!!!!!!!!?

We are getting our first puppy very soon and I wanted to get health insurance for him thru Purina and it says that I would have a $250 deductible. It says that Purina Care will reimburse 80% of eligible expenses, customer copay is 20%, after annual deductible is met. What does this mean? Like do I pay the normal vet bill until I have paid $250, and than after that I only have to pay 20% of my bill or what? Please help I'm pretty dumb when it comes to this sort of thing:) Thanks