Step 1: What decides acidity.
A hydrogen is acidic if the negative ion (carbanion) left behind after removing that hydrogen is stable. The more stable the anion, the more acidic the hydrogen.
Step 2: Locate the alpha hydrogen.
The alpha carbon is the carbon directly next to the carbonyl \((C=O)\) group. The hydrogens on this carbon are the alpha hydrogens.
Step 3: Remove an alpha hydrogen.
When a base pulls off an alpha hydrogen, a carbanion forms on the alpha carbon, which carries a negative charge.
Step 4: Stabilise the carbanion.
This negative charge does not stay on carbon alone. It shifts onto the electronegative oxygen of the carbonyl group through resonance, forming an enolate ion in which the charge sits on oxygen.
Step 5: Why this makes it acidic.
Because oxygen holds the negative charge much better than carbon, the enolate ion is quite stable. A stable conjugate base means the alpha hydrogen is easy to remove, so it behaves as acidic.
Answer: The alpha hydrogens are acidic because, on their removal, the resulting carbanion is stabilised by resonance with the carbonyl group, placing the negative charge on the electronegative oxygen (enolate ion). This resonance stabilisation of the conjugate base makes the alpha hydrogens acidic in nature.