The earth moves around the sun and as it carries out this movement the sun's rays reach the earth with a different inclination, producing cyclic variations in the temperature of our planet which we divide into four seasons.
As the rotational axis of the earth is inclined, the sun's rays do not always reach the Northern and Southern Hemispheres in the same way.
In the following visual you can observe how the rotational axis of the earth is inclined; when the North Pole is furthest from the sun, the South Pole is nearest the sun and vice versa. When it is winter in the Northern Hemisphere, it is summer in the Southern Hemisphere and vice versa.