What does Side chain do? -- What are the Q, F, G handles for?

Can someone explain to me what the side chain effect (dspmixfx ch.strip) does?
What are the Q, F, G handles for?

The side chain, e.g. of a compressor, is the detection unit where the incoming audio signal is analyzed. The 3 knobs belong to the side chain filter that allows you to increase or attenuate frequencies of the signal before it gets analyzed. This way you can change what the compressor is “hearing” and what it’s reacting to. The filter doesn’t change the main audio signal.

Q, F, and G are the parameters usually of a bell or shelf filter. The Q factor determines the “shape” of the filter, e.g. the width of the bell, F is the filter frequency and G the gain added or removed at the filter frequency.

Here you can find some more information about side chains:

I understand but there’s no effect when I turn on and off the side chain section. Compressor is still working as usual. For example if i set F to 13kHz and G to -18Db there’s still no effect but it should work like maybe de-esser.

The compressor reacts to the level created by the whole frequency spectrum. If you lower the level by reducing the gain of certain frequencies, it reacts less. To make it react more to the frequencies around 13 kHz, you need to increase the gain of the side chain filter instead of lowering it.
However, it’s not the same as a de-esser which only analyzes a selected frequency band.