What is consensus.BIP34Height? In the bitcoin source code, I am looking at chainparams.cpp. I can see on line 81 something called consensus.BIP34Height? What is that? I can't figure out what this is. 

I'm not sure what else to ask other than what is this? I can't see any mention of it in the developer reference.
The specification for BIP34 was created to fix a vulnerability where it was possible to make two transactions with the same TXID. To fix this a nonce was added coinbase transaction, where the height of the block must be encoded in the first bytes of the first transactions input. The definition is for when that soft fork became enforced.