Feedstock is a term which can cover any type of raw (unprocessed) material. In the modern energy business, it’s most commonly used to refer to a biomass source (of an type: crop, waste and so on) in the context of feeding into some downstream conversion process. The conversion process can be either directly to energy or to an intermediate energy carrier (such as biogas or bioethanol).
As examples, corn can be the feedstock into ethanol production, animal manure the feedstock into biogas production and so on.
So a biomass feedstock is a primary energy resource (ideally a renewable one) in the same way that crude oil is a non-renewable primary energy resource. Both can be used to feed into a variety of processes for energy, refined fuels, chemicals, plastics and more.