Flight modes in OpenTX are simple compared to most radios.
The settings are simple: A name (displayed on the main views), a switch to activate them, a trim setting and 2 fade in/out settings.
Yet they are very powerful, because the main settings are actually just located somewhere else: in the D/Rs and mixers.
Each of these has a flight mode selection list, that will determine in which one(s) they are active.
Most everything is controlled through dedicated mixers.
The mixers that are controlled by a flight mode will see their activation fade in/out according to the mode's settings.
As the trims can be made flight-mode specific (they are by default), using flight modes to activate things like gear or flaps allows using the separate set of trims to counter the extra drag that often causes an effect on pitch.