HODL is commonly known as the misspelled form of the word HOLD - As in don't sell, hold. Originally believed to be started by a drunk online trader by the name of  GameKyuubi, when in a drunken online rant misspelled "HOLD" multiple times.

I type d that tyitle twice because I knew it was wrong the first time. Still wrong. w/e,

Subsequently, the term has gained international notoriety and become a popular meme within the community, with thousands of references being made to it online.

HODL is an Investing Strategy and Philosophy

The term HODL has become  an approach and philosophy for many cryptocurrency investors. It's also a strategy used to 'wait out' short-term price moves and the high volatility nature of the cryptocurrency market.