You can use a fast disk like an SSD as a 2nd level cache device after ram. So reads will first check the main ram cache first, then the ssd cache disk before finally hitting the underlying storage.

a cache drive is l2arc. Brendan Gregg's post[0] explains it well, but the TLDR is that l2arc is very much like an l2 cache in a cpu. Recent and relevant items are held on a fast drive in case it will be needed in the near future. It is really an L2 cache: L1 is called ARC and is held in dram.

If I recall correctly, the extra indexes and tags mean l2arc requires more memory to work efficiently.

[0]: https://blogs.oracle.com/brendan/entry/test