What is this text effect called, and how do I recreate it?

That's a simple "(fake) inversion effect" wherre a duplicate of the number's inner shape is used as a track matte on a duplicate of the photo or even simpler yet the photo is merely sandwiched between two separate layers of the text - one with the fills at the bottom, the otehr with the stroke on the top. It's all an illusion. In the long run you may want to learn that track matte stuff, though, as it's more unviserally usable even in scenarios where you can't always arbitrarily change the layer order. The other method is simpler for beginners, of course.

Mylenium