What is an enzyme-catalyzed reaction?

A catalyst is something that speeds up a chemical reaction by reducing its activation energy - the energy required for that reaction to occur.

Enzymes are biological catalysts. Enzymes are proteins which bind to substrates (reactants), putting them in a position that enables them to more readily undergo a particular reaction (through a reduction in activation energy).

An enzyme-catalyzed reaction is a chemical reaction that is catalyzed by an enzyme.