Swap one hydrogen for carbon, and an aldehyde becomes a ketone.
Both contain a carbonyl group: carbon double-bonded to oxygen.
In an aldehyde, the carbonyl sits at the chain's end, bonded to hydrogen.
In a ketone, it's buried mid-chain, with carbon on both sides.
That hydrogen makes aldehydes oxidize to acids; ketones resist. Propanal versus propanone.