What is function of motor capacitor ?

A motor capacitor,  such as a start capacitor or run capacitor (including a dual run capacitor)  is

an electrical capacitor that alters the current to one or more windings of a single phase AC induction

motor to create a rotating magnetic field. There are two common types of motor capacitors, run

capacitors and start capacitors. The units of capacitance are labeled in microfarads (µF). Older

capacitors may be labeled with the obsolete terms "mfd" or "MFD" which can be ambiguous but are, especially

in this context, used for microfarad as well (a millifarad is 1000 microfarad and not usually seen on

motors).
Motor capacitors are used with air conditioners, hot tub/jacuzzi spa pumps, powered gates, large fans

or forced-air heat furnaces for example.  A "dual run capacitor" is used in some air

conditionercompressor units, to boost both the fan and compressor motors.

(Adopted from https://en.wikipedia.org/wiki/Motor_capacitor)