using many, often synonymous and redundant, keywords in a single HTML element, typically a headine
Packing many keywords, typically into a headline, in an effort to trick The Algorithm into thinking the page is better (more highly relevant to more search queries) than other pages which provide simple, clear, human headlines. Sadly the technique isn't entirely ineffective, although Google does seem to have tuned down the volume.
This practice is a form of information noise pollution. It's evil, and the people who do it should be ashamed. They have no shame, so that's not going to happen, of course.
The only good news is that they're skating on thin algorithmic ice. As the AI gets better at detecting this kind of nonsense, they risk having their pages fade into irrelevance.