Buying The Dip is an accumulation strategy where one buys a coin after a price decrease - the dip -, in hopes that the sell-off causing the dip was too large and the price will bounce back to a higher value.
Everyone wants to buy the dips - that is where the most money can be made -, but there should always be a valid reason to buy a coin, not just the fact that it's lost value.
Buying the dip is one form of trying to time the market. And timing the market consistently is very difficult to do.
When a coin drops significantly in price, it's very common for traders to say "Buy The Dip". Sometimes just as a joke, sometimes as serious advice, and often as a way to cope with their loss.
* All terms and definitions may update as the Cryptionary improves.