Autolysis happens when yeast cells rupture and release their internal contents into the beer after sitting on a large yeast cake for too long post-fermentation, especially at warm temperatures — the result is a rubbery, yeasty, sometimes meaty off-flavor that's genuinely unpleasant and, once present at any real intensity, doesn't age or condition out.
Time and temperature both matter, not just one
Autolysis risk builds gradually rather than switching on at a fixed day count — a beer left on a large yeast cake for many weeks at warm room temperature is at meaningfully higher risk than the same beer racked off within a couple of weeks, or left considerably longer but at cold, near-freezing temperature where yeast cell breakdown slows dramatically. Cold conditioning is a genuine mitigation, not just a clarity technique.
Why this matters more for some styles than others
A beer intended for very long aging — Barleywine, Russian Imperial Stout, strong Belgian styles — needs a deliberate racking or transfer off the primary yeast cake specifically to avoid autolysis risk over the extended timeline those styles call for, where a standard-strength beer consumed within a few months after a normal fermentation and packaging schedule rarely encounters the problem at all.