My log says “Allowed memory size exhausted”, what does that mean?

That’s PHP Fatal error: Allowed memory size of N bytes exhausted, a single PHP request tried to use more RAM than memory_limit allows, so it died with a 500. The lazy fix is to raise memory_limit, but that just hides the leak. We find the hot path, usually an unbounded collection load, a huge product import, or a runaway loop in a plugin, and cap or fix it so the limit isn’t hit again. Magento recommends at least 2G for production and 4G for some CLI tasks.