What do PCWP, PCWC, and P->S mean?
Paraphrasing from Explain Plan with Parallel Processing:


  The IN-Out column can take on the following values: PCWP, PCWC, P->S, S->P, and
  P->P.
  
  The values PCWP (Parallel Combine with Parent) and PCWC (Parallel Combine wth Child) indicate intra-operation parallelism.  For every PCWP there will be a PCWC. These pairs of operations are executed by the same set of parallel processes.                   
  
  The value P->P (Parallel to Parallel) indicates inter-operation parallelism.  This communication will between two different pairs of parallel processes. The value P->S (Parallel to Serial) always occurs in a SQL statement when the parallel processes send their data to the query coordinator.The value S->P (Serial to Parallel) occurs when a table is read in serial mode, such as a small table, and then its data is sent to an operation that is executed in parallel.


There are additional references available here and here.