It is a concept in the Bitcoin community to keep holding long term. It is a misspelling of the word "Hold".
Also said to mean "Hold On for Dear Life". It reinforces the financial concept that you don't sell in a Bear Market. You ride it until the bitter end, or the price comes back up.
The Bitcoin price has been a wild ride. Also there have been many crisis and adversities. Hodling has been a stance to keep holding no matter what problems or price crashes that came along.
To HODL - a verb that means to keep holding.
A Hodler - a person that practices the philosophy of long term Bitcoin holding.
It originated in a December 2013 post on the Bitcoin Forum message board by an apparently inebriated user who posted with a typo in the subject, "I AM HODLING."