Inverter drives motor so that its speed can be controlled by changing AC signal frequency.
Controlling motor speed, to control pump output flow for example, has better energy efficiency than controlling pump output by valve.

How inverter works?
AC signal is converted to DC signal then inverted again to modified AC Signal (variable voltage & frequency).

https://www.mitsubishielectric.com/fa/assist/e-learning/pdf/eng/FA_Equip_for_Begin_eng_Inverters.pdf

Mitsubishi freqrol inverter basic

https://www.mitsubishielectric.com/fa/assist/e-learning/pdf/eng/3-FREQROL_Basics_Fn_na_eng.pdf