Monofilament strings have a rather simple design. A certain string material, or a combination of materials, is extruded, or drawn through a geometrically shaped dye, to form a solid piece of string. The materials used can be polyester, copolymers ("soft polyester"), aramid (“Kevlar”) or PEEK polymers. This basic construction is rather tear-resistant and inelastic. Initially this provides a very "crisp" feel, which tends to decline rather quickly.
The advantages of monofilament strings are particularly relevant for “hard hitters” (tear resistance and control) and for extreme topspin players (little string slippage and good snapback effect). A special coating on the exterior or a structured surface often support their spin generating property.
On the other hand monofilaments generally transfer more shock down the racquet into the handle than a softer, multifilament string. That’s why some players only use monofilament strings for the main string in a hybrid string setup.