PCell stands for parameterized Cell, a concept used widely in the automated design of analog integrated circuits.
A PCell represents a part or a component of the circuit whose structure is dependent on one or more parameters.
Hence, it is a cell which is automatically generated by electronic design automation (EDA) software based on the values of these parameters.
This code is responsible for the process of creating the proper structure of the PCell variants based on its (input) parameters.
For the example of a physical PCell, this code generates (draws) the actual shapes of the mask design that comprises the circuit.