Ingest is a triggered ability found on certain Eldrazi creatures: "Whenever this creature deals combat damage to a player, that player exiles the top card of their library." It's a slow, grinding form of mill/exile that fuels the Processor mechanic, since Processors interact with cards your opponents own in exile.
The nuance people miss: ingest only triggers on combat damage dealt to a player—not creatures, not planeswalkers, and not noncombat damage. It also exiles cards from the affected player's library, not your own. Importantly, those exiled cards belong to that opponent, which matters for Processors and effects that return cards from exile.
In Commander, Ingest is mostly underwhelming—one card per hit barely dents a 99-card deck across multiple opponents. It's primarily relevant in dedicated Eldrazi Processor builds, where exiling opponents' cards enables payoff effects rather than serving as a real mill strategy.