Krausen is the foamy, often brown-tinged head that forms on top of actively fermenting beer — a mix of yeast cells, hop resins, and proteins carried upward by the CO2 fermentation produces. Watching krausen rise, peak, and fall back down is one of the simplest visual ways to track fermentation progress without taking a gravity reading.
A visual signal, not a replacement for a hydrometer
A tall, vigorous krausen usually signals healthy, active fermentation, and its collapse back down is a rough sign fermentation is slowing — but "rough" is the operative word; krausen height and timing vary by yeast strain, pitch rate, and temperature, so it's a useful supporting observation rather than a substitute for confirming FG with an actual gravity reading before assuming fermentation is finished.
Krausening: the same word, a different technique
"Krausening" borrows the term for a specific carbonation technique — adding a small amount of actively fermenting wort (in full krausen) to an already-fermented beer, using its live yeast and remaining fermentable sugar to carbonate naturally, similar in effect to priming sugar but using fresh wort instead of measured sugar. It's a traditional German lager technique, less common among homebrewers than standard priming sugar but a genuine, well-established alternative method.