In mathematics, natural numbers are whole numbers starting with 1 and going upward. In some fields, the natural numbers start with 0 but, ultimately, natural numbers do not include negative numbers or fractions.
Natural numbers are also referred to as "counting" numbers because people use them to count and order things. Given that, there is some disagreement amongst mathematicians about whether 0 is a natural number or not. Humans have used natural numbers to count since caveman days, and to count something, people started with 1. This is the reason mathematicians against starting with 0 claim it is not a natural number.