If you want to learn about a subject, an encyclopedia is a good start

A channel simply means a carrier frequency. Like on your TV or FM radio: you can retune to a different one to get a different station. Depending on the region, WiFi can use 12, 13 or 14 channels. That is quite a limited number, so WiFi routers quite often share the same channels. (The situation is actually even worse due to the channels overlapping, read the Wikipedia article. In practice you are stuck with only about 3 usable channels.)

If two networks share the same channel, they need to time multiplex (basically same as time sharing on your holiday property). In other words, two networks on the same channel compete for the bandwidth which may result in a slower connection. To avoid the problem, it may help to switch to a less busy channel.

How to do that is device specific. Most routers and WAPs have a web interface with a lot of options, including the channel. Whether or where such an option is in QT Mobile Hotspot, I have no idea. Never used it and do not intend to.