Altitude is how high up something is. It is usually referenced relative to mean sea level. So sea level has an altitude of 0, anything higher than sea level has a positive altitude, and anything below sea level has a negative altitude.

Altitude is the distance between an object and sea level. Thus, for example, the summit of Mount Whitney, the highest mountain in California, rises 14,505 feet above sea level. Thus the altitude at the summit of 14,505 feet.

Altitude is height. We usually measure the altitude compared to sea level. So if we're on a mountain that's 10,000 feet above sea level, we say we're at an altitude of 10,000 feet.

Altitude is basically a measure of how tall something is. You measure the distance between the point you are interested in finding the altitude of and some reference point. Usually average sea level or average ground level are used as a reference point. We use the average because there may be slight differences in “sea level” or “ground level” depending on exactly where you measure from (like if you measured from the top of a hill or the bottom of the hill). For example, if you are on an air plane and the captain says that you are at an altitude of 30,000 ft (the cruising altitude of a Boeing 737), that means that the aircraft is roughly 30,000 ft from the ground.