Why the composition of two functors is a bifunctor？

Question

I just know that the composition of two functors is in the same way as functions，but I can't understand how it can be a bifunctor.

Answer 1

The quote is stating that $$\circ: \mathrm{C}''^{\mathrm{C}'} \times \mathrm{C}'^\mathrm{C} \to \mathrm{C}''^\mathrm{C}$$ is a bifunctor: that is, it is a functor whose domain is a product of categories.