What is the "burning" process?

Coin burn in cryptocurrencies means sending some of the coins of native cryptocurrency or some other currency to a public address from which those particular coins can never be spent because the private key of such an address is unobtainable.
This public address should be available on the blockchain for anyone to review such a transaction.
Coin burn is typically done for purposes such as:
1) For rewarding the token or coin holders
2) For destroying unsold tokens or coins after an ICO or token sale