A number such as 5 or 200 cannot change its value. For example, if a person stands 5 feet away from a building, the distance variable compared to the building does not change over time. 5 feet is called a fixed amount, which is the opposite of a variable amount.
But if a person walks away from the building, the distance variable compared to the building is increasing. So it does not have the same value and we can not use the same number for it, as the distance might be 1 foot now, but 2 feet a second later. In mathematics, a variable is usually given a letter, such as x or y.
In computer science, a variable does not have to be a number—it may be a string (text) value, or a date, or an amount of money, or even an object such as a picture.