It's called by different names depending on where you get it, but it's just apple juice in slushie form, and I love it.

So simple but so great on a warm day. I don't care for the other slushie flavors because they are fake and sugar added, but the apple one is just 100% Minute Maid Apple Juice.
In Toontown, you can get them at the cart by Goofy's house.
The Eidelweiss cart near the Matterhorn has them.
Maurice's has them, but I hate the foam and get it without if I get it there, but I think it's pricier because of the foam if I recall correctly.
The Cozy Cone has them as "Red's Apple Freeze" but again with foam and same on price.
In Bug's Land, you can get the at the cart near Heimlich's train.
There might be other areas, but those are the ones that come to mind.