The relative atomic mass (Ar) of an element compares the mass of its atoms with the mass of other atoms. It is an average value for the atoms of the element as they naturally occur, so it takes into account every isotope.
Many elements are a mixture of isotopes with different mass numbers. Each isotope has a whole-number mass number, but the average depends on how much of each isotope is present. This is the abundance of the isotope. The average mass is not usually a whole number.
Chlorine is a good example. About three quarters of chlorine atoms are chlorine-35 and about one quarter are chlorine-37. The average is closer to 35 than to 37, so the relative atomic mass of chlorine is 35.5, which is not a whole number. Boron has isotopes with mass numbers 10 and 11 and a relative atomic mass of 10.8, which shows that boron-11 is the more abundant isotope. An element with only one isotope has a relative atomic mass that is a whole number or very close to one.