What is a 9 channel RC transmitter?

It's just what it sounds like: a RC transmitter that sends nine channels of data to the model.

What it DOESN'T say is why you'd want that many. You have a plane...say it's a P-38, which has two engines. You need at the very minimum:

rudder channel

elevator channel

aileron channel

throttle channel

Real P-38s have retractable gear so throw in a fifth channel for the retracts. And for many years, that's all you had.

Then you might want a channel for flaps, some guys split the ailerons into two channels, you might want to make the plane drop model bombs so that's another two channels (one for the doors, one for release)...already you're at nine channels and you haven't added your landing lights, smoke system or whatever other things you might want on the plane.