In the periodic table it lists chlorine what does the number 35.453 mean?

The number that's in the position where chlorine has "35.453" is the average Atomic Mass in atomic mass units for that element assuming the various isotopes are present in their "natural" ratio. The two common isotoped of chlorine are chlorine-35 and chlorine-37; since in nature there are about 3 35Cl atoms per 37Cl atom, the ratio works out to be distinctly non-integral (also contributing slightly to the non-integral average mass is the fact that the atomic masses of the two isotopes are not EXACTLY 35 and 37).