What does it mean when two bunnies do this?

It means your older rabbit is likely to injure the smaller one. Rabbits are very territorial, especially does. Separate them now. You couldn't keep these two together anyway, as they would breed as soon as they're able to. Bucks and does can never share a cage; they always need separate housing unless you plan to breed. And even then, the doe is taken to the buck's cage (NEVER the other way around because she could kill him) just long enough for mating to take place and then she's returned to her own cage. Please do some research on keeping rabbits as pets so you don't have some bad outcome because of not knowing basic care info.