For your typical "hodler", a dip is simply just a drop in price. I'm sure only a very small percentage of the "hodlers" are actually decent traders, so if someone says "buy the dip", then chances are, the person just said it just because the price dropped a certain percentage. Always remember that bitcoin and the cryptocurrency markets are hugely unregulated, so expect a lot of amateur "investors" in this space. Not to mention that "buy the dip" is also being used as a meme nowadays since the bear market started, when people thought price $x was the floor, but it continued to drop lower. Buy the dip!
So refreshing actually having a good well-thought question here in Bitcointalk.