IFR stands for instrument flight rules – the set of rules that govern aircraft that fly in IMC, or instrument meteorological conditions. In general terms, instrument flying means flying in the clouds. More specifically, IMC is defined as weather that is “below the minimums prescribed for flight under Visual Flight Rules.”
It’s called instrument flight because the pilot navigates only by reference to the instruments in the aircraft cockpit. Flying in the clouds (IMC) requires an IFR flight plan and an instrument rating.