As the Earth revolves around the Sun, and as the Earth rotates on its axis, which is tilted 23.5 degrees, the sunlight reaches the planet in different degrees, and that is why we have different seasons around the world.
The solstice is when the sun is at its highest or lowest point in the hemisphere, and from that day, the days of sunlight are longer or shorter, in the winter and summer solstices.
The autumn and spring equinoxes are the days when the axis of the Earth is as straight as possible and the sun's rays fall on the equator. While in one hemisphere it is spring, in the other it will be autumn.