The fundamental principle behind the flight of a quadcopter is Newton's Third Law of Motion, which states that for every action there is an equal and opposite reaction. The propellers of a quadrocopter push air down. This causes an opposite reaction called thrust that pushes the quadcopter upwards against gravity. Drones are also known as unmanned aerial vehicles (UAVs), which can operate without pilots and include both airplanes and helicopters.
An unmanned aerial vehicle (UAV), commonly known as a drone, is an aircraft with no pilot, crew, or human passengers on board. Unmanned aerial vehicles were originally developed during the 20th century for military missions that were too boring, dirty, or dangerous for human beings, and by the 21st century, they had become essential assets for most armies. As control technologies improved and costs declined, their use expanded to many non-military applications. These include aerial photography, precision agriculture, wildfire monitoring, river monitoring, environmental monitoring, police and surveillance, infrastructure inspections, smuggling, product delivery, entertainment, and drone racing.
Drones use rotors for propulsion and control. You can think of a rotor like a fan, because they work pretty much the same way. Of course, all forces come in pairs, meaning that as the rotor pushes air down, the air pushes the rotor up. This is the basic idea behind lift, which boils down to controlling upward and downward force.
The faster the rotors turn, the greater the lift and vice versa.