The Earth experiences seasons because its axis is tilted, and as it rotates, sometimes a region is tilted towards the sun. In the northern hemisphere, when the Earth is closest to the sun, the region will experience winter, and when it is furthest away, it will experience summer.
The axis of the Earth is an imaginary pole that goes from the top to the bottom, and this pole does not stand up straight. The Earth makes one complete spin around the pole every day, which is why there is day and night. The tilted axis always points in the same direction, so during different times of the year, different parts of the Earth will experience the Sun's direct rays.