Density refers to the amount of mass per unit volume. In the case of air and helium, it is the measure of how much space a certain amount of air or helium takes up.
Density and weight are indirectly related because density is a factor that affects weight. The denser an object is, the heavier it will be for a given volume.
Helium has a lower density than air because it is a lighter gas. Helium atoms are smaller and have less mass than the nitrogen and oxygen molecules that make up the majority of air.
Temperature affects the density of air and helium because it changes the speed at which the molecules move. As temperature increases, the molecules move faster and spread out, decreasing the density. As temperature decreases, the molecules slow down and become more tightly packed, increasing the density.
Understanding the density of air and helium is important in various fields such as meteorology, aviation, and gas production. It is also crucial in determining the lifting capacity of helium balloons and in the design and performance of air and helium-filled objects such as blimps and airships.