Step 1: Location of alpha-hydrogen atoms.
Alpha-hydrogen atoms are bonded to the carbon directly adjacent (the alpha-carbon) to the carbonyl $C{=}O$ group. Their acidity depends on the stability of the anion (conjugate base) formed after their removal.
Step 2: Formation of the resonance-stabilized enolate ion.
When a base removes an alpha-H, the resulting carbanion is immediately stabilized by delocalization into the adjacent $C{=}O$: \[ RCH^{-}{-}CHO \;\leftrightarrow\; RCH{=}CHO^- \] The negative charge is spread between the alpha-carbon and the electronegative oxygen atom.
Step 3: Why resonance stabilization confers acidity.
A more stable conjugate base (enolate ion) means the proton is released more readily. Since ordinary $C{-}H$ bonds do not produce resonance-stabilized anions, alpha-hydrogens of aldehydes and ketones are distinctly more acidic than typical alkyl C-H bonds.