Why do the masses of each element have a decimal if particles each way 1 amu?

If you are asking about the average atomic mass, the number under the element, then it is "the sum of the masses of it's isotopes." You should know what isotopes are, (if not, it's when an element can have more or less neutrons than other atoms of the same element) so what it means is that the number is the average amount naturally found in nature ( or naturally produced). For example, carbon's number is 12.0107. Since the average atomic mass number is so close to 12, it must mean that carbon's most abundant iostope is Carbon-12. The decimals mean there are Carbon-13s in nature, but not as abundant as Carbon-12. In some cases, there can be as many Carbon-11s(which isn't stable, but this is an example) as Carbon-13s, so the number is on 12, so you have to pay attention to which isotopes are listed as natural.