Hodl(ing) has become a buzzword in the field of crypto currencies that means nothing else but "HOLD". It originated from a spelling mistake of hold, the English word for hold. As HODLER one calls people, who keep their crypto currencies (Bitcoin, Monero, Dash, Ripple etc.), thus hold, and hope that these will be worth more in the future again. Many people "hodle their coins" and hope to either make a profit or at least compensate their losses a bit.
Many people know the term "hold" as a valuation element for shares. If the market does not have a clear opinion about the company and the price often fluctuates, rating houses recommend their customers to hold the shares to wait and see. This principle is often used by people who have previously bought for more money and simply speculate on higher prices. If the signals are clearer or important company announcements are made, Hold can then become Buy or Sell.
By the way, in poker there is also the term HOLD, you want to wait and see what the others do and not invest more.
The question is whether HODLing would have come into being at all if prices hadn't gone into such a severe slump in 2018? Many people got in at that time and are now sitting on crypto currencies. These people wait before they consider selling or are so convinced of the concept that they use crypto-currencies as long-term speculative objects (similar to gold). On the one hand the mass of persons is a security factor for further exchange rate losses (who does not sell stabilizes unconsciously the course) on the other hand it could come in case of rising courses in addition, to numerous sales (which would brake the course rise).
