Feed freshness
Feed freshness is how recently a product feed’s contents were rebuilt, and therefore how likely its prices and availability are to be right at the moment something reads them.
A feed is usually the cheapest source of a catalogue, so it is frequently the one used. That makes staleness worse than absence: an absent feed sends an agent to your pages, slowly but accurately, while a stale one gives a fast confident wrong answer.
Nothing about a stale response suggests checking further. The file resolves, it parses, and an uptime check on the URL passes indefinitely while the data inside it ages.
Why it matters to a store
The failure is silent in the direction that costs you. A price published too high loses comparisons you would have won, and no analytics record a comparison you were dropped from.