What does this comment in Chainparams.cpp mean?
The original 50 BTC coinbase is not spendable since it was not included in the UTXO set. Any transaction trying to spend it would be invalid, since the previous output from the coinbase tx does not exist in the utxo set.

The hex string is an encoding of The Times 03/Jan/2009 Chancellor on brink of second bailout for banks, along with the regular coinbase params. It is essentially a marker that proves that the chain was not premined before 03/01/2009 by including the headline from a paper on that day.