What is a monochromatic light

As the name implies, light of 1 color (frequency) only.

There is no such thing because that implies a light shining for infinite time. But you can get pretty close. A laser puts out almost monochromatic light, as does an emission spectrum from a low-pressure excited gas, e.g. sodium lamp (actually 2 yellow wavelengths very close together)

Light with a single frequency (photons all with the same energy), or at least so close together in energy that no-one notices.

A light with single wavelength.