How works the ePWM block?
The ePWM block is part of the Embedded Coder Support Package for Texas Instruments C2000 Processors for Simulink. You'll first have to install this support package in Simulink. See this link for that.

This block is for using the ePWM module (enhanced PWM module) in the compatible Texas Instruments microcontrollers. PWM module is a common module found in most MCUs for generating PWM signals. The MathWorks page shows the following families of TI microcontrollers supporting this feature:

C280x
C2802x
C2803x
C2805x
C2806x
C2833x
C2834x
F28M3x
F2807x
F2837xD
F2837xS  

If you haven't deployed Simulink models to development boards before, this video will help you deploying Simulink models to TI C2000 series microcontrollers.

You will find more information on how it works in the ePWM module datasheets provided by Texas Instruments. A good starting point can be this: http://www.ti.com/lit/ug/spru791f/spru791f.pdf