The document discusses the density of different number sets. It explains that the real numbers and rational numbers are dense, meaning there is always another number between any two numbers. The whole numbers are not dense as there is no whole number between 1 and 2. Irrational numbers are also dense since you can always find a number halfway between any two irrational numbers by adding them and dividing by 2. Finally, it states that as rational numbers and irrational numbers are dense and real numbers are the intersection of these sets, real numbers must also be dense.