The L2ARC is the 2nd Level Adaptive Replacement Cache, and is an SSD based cache that is accessed before reading from the much slower pool disks. The L2ARC is currently intended for random read workloads. This statistic shows the read and write byte rates to the L2ARC cache devices, if cache devices are present.
This can be useful to check during warmup. The write bytes will show the rate of L2ARC warmup of time.
Also see Cache: L2ARC accesses.