You know how Stannis offers to make Renly his heir until he has a son? Well isn’t he his heir already, given that a male already inherits before a female?

I think it’s the difference between heir presumptive (who can be displaced if another heir comes along in the mean time before the current ruler dies) and heir apparent (who can’t). 

It’s also about a pardon – from Stannis’ perspective, Renly has forfeited his heir status by rebelling against him. So he’s offering it back.