If the weather is good enough, pilots can fly by looking out the window and avoiding hazards. There are some rules involved about how close a pilot can come to clouds, restricted or controlled airspace, and ground structures, and those rules are referred to as visual flight rules. Most small plane flying is conducted under VFR. A pilot might also say that the weather is VFR, meaning that the weather is clear enough that flight under visual flight rules is legal, or that an airport is VFR meaning that the weather at that airport is VFR.
Set of Federal Aviation Administration (FAA) rules, guidelines, and procedures that apply to aircraft when a pilot is conducting flight with visual reference to the ground.