A mash-up is when two (or more) segments of stuff – in this case two of the great movie franchises of the 20th Century – are combined to form something else altogether.
Another form a mash-up can take is superimposing one thing on another, thusly:
I have no idea who created these but they’re a genius. And isn’t interesting how the women look a whole lot better than the guys?
This isn’t a mash-up, but it’s too cute to be ignored. Grrrrowrrrrr!