What is cardio?

Unfortunately, "cardio" is a slang word that doesn't mean much. It's short for "cardiovascular", and folks use it to refer to anything from doing a few jumping jacks to preparing for a marathon.

The right term is "aerobic exercise". Aerobic exercise involves the heart and lungs, and the slow-twitch muscle fiber which gets energy from the oxygen/glucose metabolism. Ideally, it's a steady-state affair where the production of energy is balanced by your ability to remove wastes..... So you can work at an "aerobic" level for a long time. Hours, if you're fit.

Aerobic exercises are anything that gets your heart rate into the aerobic training zone, 65-75% of your max. So....Running, swimming, cycling, stationary machines, aerobic dance... Etc.

sports like tennis are not aerobic... They are stop-start affairs. Aerobic exercise is steady effort.

Normally that is short for cardiovascular, and usually refers to doing exercise that improves the human cardiovascular system.

Meaning you can uptake more oxygen and do better at endurance sports.

An exercise that raises the heart rate for an extended period of time.