How are subspecies defined, in contrast to species?
Generally speaking, different species can't interbreed. For example, a giraffe can't mate with a gorilla.

Occasionally, very closely related species do interbreed - e.g. lions and tigers breeding in zoos. But the offspring are generally infertile.

Subspecies can interbreed, but they seldom do simply because they're separated from each other.

For example, consider the brown bear and the wolf, each of which have vast ranges embracing a wide variety of habitats. A subspecies adapted to a montane environment isn't likely to breed with a subspecies adapted to a desert environment. Of course, it would have been virtually impossible for a Eurasian wolf subspecies to interbreed with the wolf subspecies that lived in Newfoundland prior to its eradication.

The separation doesn't have to be geographic. Subspecies can sometimes evolve physical or behavioral differences that inhibit them from breeding with other subspecies.