HODL refers to HOLDING coins at all times and refusing to sell them, even when prices are going down ... a strategy used by those who don't know how to trade or simply don't want to do it.
The term HODL was born when a member of the bitcointalk forum made a post while drunk, making this epic TYPO error ... and the rest is history. Check the original thread to see how it all started : https://bitcointalk.org/index.php?topic=375643.0