bigc is Clustrix's garbage collection process that cleans up undo logs needed to rollback running transactions. Clustrix uses multi version concurrency control (MVCC) and as part of this must to keep the system state of a transaction while open, and cannot clean up past the oldest running transaction. Once the oldest running transaction is commited "bigc" cleans up the various undo logs.
The term "pinning bigc" means that the process bigc is being held in state due to a long running transaction that hasn't finished. This can cause the various undo logs to grow, in some situations quite large.
To learn more about bigc and garbage collection please see the documentation: Concurrency Control