Most elements are a mixture of isotopes, and the isotopes have different masses. The relative atomic mass, Ar, of an element is an average value that takes account of the abundance of the isotopes of the element. An isotope that is more abundant has more effect on the average.
To calculate it, multiply each mass number by its percentage abundance, add the results, then divide by 100. Chlorine is 75% chlorine-35 and 25% chlorine-37. So Ar = (35 × 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 35.5.
The answer is not a whole number because it is an average. It is also a useful check: 35.5 is closer to 35 than to 37, which tells you that the lighter isotope, chlorine-35, is the more abundant one. If the two isotopes were equally abundant, the relative atomic mass would be exactly halfway between their mass numbers.