HODL is a typo of "hold" from a 2013 BitcoinTalk post titled "I AM HODLING," which became a cult. It means hold the asset no matter what, don't panic, don't sell on dips.
Sometimes backronymed as "Hold On for Dear Life" — but the original was just a misspelling.
Example#
"BTC dropped 20% again. NBD, hodl till the moon."