A Start Capacitor is used to store and provide energy that can be used to generate the initial torque sufficient to start the engine. It is then removed using a centrifugal switch after a predetermined speed has been reached, which is normally 75% of full speed. After that, it works more efficiently with a run capacitor.
They generally have a higher capacitance than run capacitors, largely in the 30uF to 120uF range. The start capacitor provides an immediate electrical push to get the motor rotation started. Without a start capacitor when the voltage is applied, the motor would just hum.
For example, in an air conditioning system, a start capacitor is used to generate an initial starting torque. While a run capacitor is used to turn the fan motor to blow the condensed air created by the system.