Earth has a 23.5degree tilt relative to the orbit, and that means the Northern Hemisphere, so one hemisphere of the earth receives more direct sunlight than the other. Let say you turn on a flashlight, and you aim it at an angle to the ground, the further the ground from the flashlight, the dimmer it is. The sun is the flash light in this case, and therefore the Hemispheres have opposite seasons. For example, I assume that you live in the U.S. and it is still winterish here, however in Brazil, it is now summer in Australia. The seasons are more extreme when you are closer to the poles, and when you are closer to the equator, there will be lesser seasonal variation.
https://spaceplace.nasa.gov/seasons/en/
