The ESH is is a shelf controller module for the DS14 disk shelves (which we've all but stopped selling). Each shelf requires two modules. They came in ESH, ESH2 and ESH4 (1Gb/s, 2Gb/s, 4Gb/s). The most recent versions of Data ONTAP only support ESH2 and ESH4 modules. As Shaunjurr mentioned, they managed the Fibre Channel Arbitrated Loop and mediate I/O requests between the storage controller and the disk shelf. You can think of them as network hubs for disk shelves.
A quick search of the NetApp Support website didn't turn up a document specifically for the ESH (it's considered a subcomponent, which rarely have separate documentation), but most of what you need to know is in the DS14 shelf guide.
I too have some questions on this topic. I am a NetApp newbie, and picked up a DS14 with dual X5501A LRC adapter to play with. What would I need to connect this to my system? And what the is the difference between LRC and ESH?
LRC is out of support from hardware point of view and is not supported by Data ONTAP as of 7.3. You would need optical multimode cable; LRC side is SC, filer side is usually LC (I do not remember SC adapters). There were several different types of LRC modules – for direct connection to controller and for connection to another LRC. You need the former. You can’t mix LRC and ESH on the same loop.
One of the downsides of LRC as compared to ESH was that one bad disk could cause the whole loop to fail, whereas ESH identifies bad disks and effectively bypasses them. If you intend to use your DS14 shelf for production data then I recommend purchasing some ESH modules, you should be able to pick them up pretty cheaply but just make sure you download the latest FW for them from NOW.