I think the man pages explain them best
lockd - The rpc.lockd program starts the NFS lock manager (NLM) on kernels that don't start it automatically. However, since most kernels do start it automatically, rpc.lockd. is usually not required. Even so, running it anyway is harmless.
statd - It is used by the NFS file locking service, rpc.lockd, to implement lock recovery when the NFS server machine crashes and reboots.