ester reduction

Reduction means adding hydrogens (or, more precisely, electrons that arrive as hydride) and stripping away oxygen. Reduce an ester and you tear off the alcohol part and turn the carbonyl all the way down to a -CH2-OH group, converting an ester into a primary alcohol. It is one of the standard ways to make an alcohol from an acid-derived building block.

The workhorse reagent is lithium aluminium hydride, LiAlH4, a powerful source of hydride (H-). Reducing an ester R-CO-OR' takes two deliveries of hydride. The first hydride adds to the carbonyl carbon, the tetrahedral intermediate collapses by expelling the -OR' group (as an alkoxide, the leaving group), and an aldehyde forms transiently. But LiAlH4 is so reactive that it immediately reduces that aldehyde further: a second hydride adds and, after aqueous workup, you get the primary alcohol R-CH2-OH. So one ester gives one primary alcohol plus the alcohol R'-OH that was the original -OR' group.

Note what you cannot easily do: with LiAlH4 you cannot stop cleanly at the aldehyde, because the intermediate aldehyde is more reactive than the ester and gets reduced before you can intervene. (Milder reagents like DIBAL-H at low temperature can stop at the aldehyde, but that is a different tool.) This same hydride chemistry reduces carboxylic acids to primary alcohols and amides to amines. The catch worth remembering is that LiAlH4 reacts violently with water and protic solvents, so reductions are run in dry ether or THF and only then carefully quenched.