hydride reduction
To reduce a carbonyl is to turn its C=O into a C-OH, converting an aldehyde into a primary alcohol and a ketone into a secondary alcohol. The most common way to do this in the lab is to deliver a hydride ion (H-, a hydrogen with its electron pair) to the carbonyl carbon. The two everyday reagents are sodium borohydride (NaBH4) and lithium aluminium hydride (LiAlH4).
Mechanistically it is nucleophilic addition where the nucleophile is hydride. A hydrogen atom carrying a pair of electrons attacks the electron-poor carbonyl carbon; the C=O electrons shift onto oxygen to give an alkoxide; then a separate workup step (adding water or dilute acid) protonates the alkoxide to the alcohol. Note the two phases: the metal hydride adds the H to carbon, and only afterwards does aqueous workup add the H to oxygen. Each B-H or Al-H bond can deliver hydride, so one formula unit can reduce several carbonyls.
The two reagents differ sharply in strength, and choosing between them is a real skill. NaBH4 is mild and selective: it reduces aldehydes and ketones but leaves esters, acids, and amides largely alone, and it is safe enough to use in water or alcohol. LiAlH4 is a powerhouse that reduces almost every carbonyl-containing group (including esters, acids, amides, and nitriles) down to alcohols or amines, but it reacts violently with water and must be used in dry ether or THF. A useful caveat: neither reagent reduces an isolated C=C double bond, so you can reduce a carbonyl while leaving an alkene untouched.
NaBH4 in methanol reduces cyclohexanone to cyclohexanol cleanly at room temperature; the same job with LiAlH4 would need dry ether and a careful aqueous quench afterward.
Hydride delivered to carbon, then aqueous workup protonates oxygen: aldehyde to 1 alcohol, ketone to 2 alcohol.
The H on oxygen comes from the workup (water/acid), not from the hydride reagent itself; the hydride supplies only the H that goes onto carbon. And neither NaBH4 nor LiAlH4 reduces an isolated alkene.