Fehling's reagent is and aqueous solution Copper (II) Sulphate, which in normal conditions is deep blue in colour. It can be used to test the presence of an aldehyde group in a compound, such as formaldehyde (HCHO, who's aqueous solution is also called formalin).

When Fehling's reagent is heated with formalin, a brick red precipitate is formed.
The precipitate is Cu2O by the way. The most popular use for Fehling's reagent is to distinguish between aldehydes and ketones.