What causes the phenomenon of seasons on Earth

The seasons are caused by the tilt of the Earth's rotational axis away or toward the sun as it travels through its year-long path around the sun. The Earth has a tilt of 23.5 degrees relative to the "ecliptic plane" (the imaginary surface formed by it's almost-cicular path around the sun).

The rotational tilt of earth on its axis towards or away from the sun, which can result in changes in weather and henceforth seasons.