What's the Flaps channel mean?

Using QGC, Radio Tab, there’s a option named Flaps channel, What’s this option used for?

Flap mean Flap。 I am not speak english.

I presume you are working on a fixed wing configuration of PX4?

If so, it will be the radio channel connected to the flaps, if any, on your vehicle. YOu can find out about flaps here: https://en.wikipedia.org/wiki/Flap_(aeronautics)

These control the lift at a particular speed. They aren’t tied to yaw, roll, thrust, so you need a separate physical control on your transmitter for them.