What does it mean?
It is mentioned in the Cryptonote White Paper https://cryptonote.org/whitepaper.pdf section 6.2.2. (Please note that the formula quoted there is for the Penalty not the NewReward). This is a fundamental difference from Bitcoin. I covered this issue further back. https://bitcointalksearch.org/topic/m.13591241. Also the tail emission is needed here since without a base emission to penalize the penalty function will not work. Most people think of Monero as an Anonymity / Privacy / Fungibility coin, but this blocksize scaling issue is critically important.