How does secondary insurance work?

Forgive me if I sound incredibly ignorant, but I have only been married 4 years, and we've always just had my husband's insurance. I'm thinking about talking to my employer about insurance, but I'm not sure if I'll be able to get it or not. So... UHC will pretty much pay everything except the 250 deductible, and then I will have 1500 left out of pocket. So how does secondary insurance work? Do they come in and cover that amount that is left? Like for example: if surgery was covered at 80% would that leave 350.00 to pay out? Does this all just kind of depend on the insurance coverage? I'm just clueless how this all works. Thanks, Michelle