What it actually is
Cramming the same search phrase into a page over and over, sometimes in white text on a white background.
Why it blows up
Google stopped counting repeats about twenty years ago. Now it reads like spam to people and to Google, and both leave.
The white-hat version
Say the phrase once in the title and once early on, then answer the question properly.
The honest way, explained properly: SEO, dumbed down.