What is Chemoselective reaction?

Example 48.4 (A reductant that chooses)

Sodium borohydride, $\ce{NaBH4}$, reduces the carbonyl group of aldehydes and ketones into an alcohol group, but leaves esters untouched. On a molecule that carries a ketone and an ester, only the ketone changes:

A stronger reductant, lithium aluminium hydride $\ce{LiAlH4}$, would reduce both groups: it is not chemoselective here.

Example 24.8 (A keto ester)

Ethyl 4-oxopentanoate, $\ce{CH3CO(CH2)2COOEt}$, with sodium borohydride gives ethyl 4-hydroxypentanoate: only the ketone is reduced. With lithium aluminium hydride, both groups are reduced: pentane-1,4-diol (and ethanol). To reduce only the ester, the ketone must first be protected as an acetal (Chapter 23).