Monofilament is a macro category of tennis string composed, as the name suggests, of a single synthetic filament (usually polyester or polyamide).
They can be produced in different shapes (round, triangular, square, etc.), surfaces (smooth, rough, twisted, etc..) and gauges (from about 1.10 mm to 1.40 mm).
They are mainly used by professional players who mainly exploit their control, spin and “durability” qualities (intended as resistance to breakage).
They can be combined with other types of strings (mutifilaments, synthetics and natural gut) to make hybrids.
The nature of these strings limits the number of breakages but it is a good idea (and extremely recommended to avoid physical problems) to replace them after about 10 hours of use (depending on the string).
If you want to know more about monofilaments I invite you to visit Stringpedia where you will find, in addition to a list of strings in constant evolution, the results of instrumental tests conducted on each string.