Process is a tool used to determine which value(s) for the “stage” or “status” Field the record type applies to. A process contains a series of stages followed by an opportunity throughout its lifecycle. This is an additional step required to create record type for Lead, Case, Opportunity, and Solution objects. This step will specify at what stage of life the record is in order to define the proper page layout.
Page layout and fields requirements are different depending on the life progress status of the record.
Ex: The page layout may be different if an opportunity is at the detection stage or at the negotiation stage.