A refractometer is a small optical instrument that estimates gravity from how a few drops of liquid bend (refract) light passing through them — fast, needs only a tiny sample compared to a hydrometer, and genuinely useful for quick mid-brew-day checks where drawing off a large hydrometer sample would be wasteful.
The trap every new refractometer owner falls into eventually
A refractometer reads pre-fermentation wort accurately, but it reads fermenting or fermented beer inaccurately if used the same simple way — alcohol changes how light refracts differently than dissolved sugar does, so a raw refractometer reading taken on partially or fully fermented beer needs a specific correction formula (accounting for both the original gravity and the presence of alcohol) before it means anything close to a real final gravity. Reading a fermented sample on a refractometer without that correction and treating the raw number as FG is one of the most common, and most consequential, homebrewing measurement mistakes.
Where it genuinely shines
Checking pre-boil or pre-fermentation gravity, dialing in a mash efficiency estimate mid-brew-day, or getting a fast read on juice or must for cider and mead — anywhere the sample is still sugar-only rather than partly converted to alcohol — is exactly where a refractometer's speed and tiny sample size are a real, uncomplicated advantage over a hydrometer.