This means that gas changes its shape when you put it in another container. Take the example of air which is a gas. If you fill air inside the balloon, it takes the shape of the balloon. If you pump air into a shopping bag, or a beach ball, it will take the shape of these objects.
Gas has no certain shape, however its volume can change with differences in heat and pressure.
A gas has no shape and volume because of the amount of energy it has. When you look at molecules of a gas, they are always bouncing off of each other, not really forming bonds with each other. Liquids and solids have less energy so they form bonds, even if they are weak ones.
It is because as the atoms spread out, the amount of empty space in the gas increases.