Buying dips is a sound strategy in a bull market and a ruinous one in a bear market. The phrase became a meme because it worked for years and then, for many assets, stopped working all at once.
A dip buy with a defined stop-loss is a pullback trade. A dip buy without one is a bet that the trend has not changed.
Example: an index has bounced from its 50-day moving-average six times in a year. Buying the seventh touch is buying the dip; the difference between traders is what they do if it breaks.
Related: pullback, bull, dead-cat-bounce, moving-average