The equator is an imaginary line running around the widest part of the Earth, halfway between the North Pole and the South Pole. So if you are north of the Equator, you’re in the Northern Hemisphere. And if you’re south of the Equator, you’re in the Southern Hemisphere. North America, Europe, Africa and Asia are mostly in the Northern Hemisphere. South America and Australia are mostly in the Southern Hemisphere.
Other planets like Mars and Jupiter also have equators, which also run around their widest part. Generally speaking, the equator is the hottest part of a planet, and the north and south poles are the coldest. But that isn’t always true.