What is the red spot on Jupiter?
Short answer: It's a storm.
Long answer: It's a high-pressure region in the atmosphere of Jupiter that has existed for at least 200 years, probably longer.
The first observation of the Great Red Spot could have been in 1665, around fifty years after the invention of the telescope. This would make the storm at least 350 years old. Occasional observations may have been made in the following years but they are hard to confirm. It wasn't until 1830 that regular observations began being recorded.
Technically speaking, the red spot is a persistent high-pressure region producing an anticyclonic storm. It is located 22° south of Jupiter's equator. It is one of many similar storms on Jupiter and other gas giant planets. Jupiter's red spot is definitely the best-known of these storms.
What created the red spot?
Unfortunately we don't understand the red spot very well. Scientists are trying to figure out what caused the storm to form and how it keeps going for so long. They are investigating things like how ultraviolet light and cosmic rays might affect the chemical composition of Jupiter's clouds.
As for what makes the red colour, it could be solar radiation reacting with ammonium hydrosulfide. We don't really know yet.