what is Primary Interop Assembly (PIA)?

In Microsoft's words:
"Like any other managed assembly, an interop assembly is a collection of types that are deployed, versioned, and configured as a single unit. However, unlike other managed assemblies, an interop assembly contains type definitions (not implementation) of types that have already been defined in COM. These type definitions allow managed applications to bind to the COM types at compile time and provide information to the common language runtime about how the types should be marshaled at run time. "

A PIA is typically a "wrapper dll" which exposes functions in a non-managed dll, allowing your managed application (.net application) to access functionality within the non-managed dll.