It is an acronym for Hold On for Dear Life, it is often found in MEMES on the internet. A Hodler does not plan to sell in the foreseeable future.
The story behind the acronym dates back to December 2013 on a bitcointalk forum, when a drunken user named GameKyuubi initiated a post and wrote, "I AM HODLING". This line has become a very popular internet meme and it is used by many investors to not sell their cryptocurrency even in case of price fluctuations.
Of course, this is not an attitude to follow, you should know how to sell when it's time or avoid investing in Shitcoins .