When you ingest something, you swallow it or otherwise consume it. If you don't ingest enough iron, you'll feel tired and weak and you'll look pale.
Trees ingest carbon dioxide, and humans ingest the oxygen that trees in turn produce. We also ingest a lot of other things, like French fries, ice cream, and root beer. You can also use this word to mean "take in information," like when you ingest the details of your history book. Ingest has a Latin root, ingestus, "poured in," from in-, "into," and gerere, "to carry."