The seasons are caused by the Axial Tilt of the Earth
The Earth's axis is an imaginary line through the centre of the Earth, from the top to the bottom. Earth spins around this axis once each day. The Earth's axis tilts towards the Sun by around 23°. This is called the Axial Tilt of the Earth.
The Earth completes one orbit of the Sun each year. As Earth moves around the Sun, different parts of the planet are tilted towards the Sun. When it's summer north of the Earth's equator, the northern part of the Earth is tilted towards the Sun. This part of the Earth gets the most direct light from the Sun. This makes it hotter. At the same time, the southern part of the Earth is tilted away from the Sun. It gets less direct light from the Sun. This produces the cold winter season.