helium

[wintery]

what property of helium allows the airship to float

[dipee65]

Helium is lighter than air

[Cumaean]

Less dense than air, to be slightly pedantic. Helium has a molecular mass of 4, air of (roughly) 28.8, and the densities of gases are approximately proportional to the molecular masses. In principle, any less dense gas will do to float a balloon (even pure nitrogen, although the atmosphere is about 80 per cent nitrogen), but the great advantage of helium is the huge difference between the two densities, which translates into the ability to get "dead" weight such as the airship's structural components, crew and cargo off the ground.

[Muppit]

What they are both trying to say is that the helium within an airship would create a buoyancy.

if something is buoyant, it is less dense than whatever it is in, and will tend to move upwards. This can be used to provide an upward (or lifting) force. In the case of an airship, the lifting force created by the buoyant material is used to lift the ship.

When the lifting force is equal to the downward gravitaional force this is called neutral buyancy.