ArcStat is a graphical display of the efficiency of the ZFS ARC (adaptive replacement cache).
From the top, first you will see some of the current values. Perhaps the most interesting one is the current size, which shows how much memory is currently in use by the ARC.
Next are four bars showing the effectiveness of the cache. (Note that 0% is also shown if there's no activity.)
Below that are a set of pie charts, showing the distribution of cache hits and misses. On the left are the totals since boot; on the right are current values. Again, with no activity the current pie chart will disappear.