The definition of "Viscosity" is the resistance to flow of a liquid (normally).A high viscosity means that the liquid will not flow easily - imagine treacle.
A low viscosity means that the liquid flows very easily - imagine water or gasoline.A viscometer is a device for testing the viscosity.
Viscosity is a fundamental property and has units of mm2/s, or mPa.s, depending on the measurement tactic employed. Temperature has a big influence on the viscosity of a liquid. When the temperature is lowered the viscosity increases. That is why your car engine labours and doesn't start as well in the cold winter months - the oil is more viscous when it is cold.