Could you give more details about where you heard this expression? It is probably some kind of bill, but the meaning may depend on where the speaker is from, and possibly whether the context is modern or in the past.
My understanding of 'tab' is a (usually informal) account of how much you owe someone. For example, a poor family who is unable to pay for their food from time to time might have a 'tab' at the local grocer's store. The shopkeeper will supply them with goods that they need and add the price to their 'grocery tab'. When they get some money, perhaps when they're paid at the end of the month, they'll pay their tab (settle their account).