"Flying by the seat of your pants" means to do or take action without a plan, to go by feel, to make decisions in the moment.
This phrase dates from the early years of aviation when airplanes had few navigational aids and flight control systems. The pilots relied greatly upon their own senses to gauge the plane's position, performance, and to find their way. They flew by experience, intuition and feel. With the seat being the largest point of contact between the pilot and the plane, it was from the seat of his or her pants that the pilot could feel how the airplane was reacting to his or her controls and how well the engine was operating.
Can you think of a time that you were flying by the seat of your pants?