Power

Consider this diagram showing light passing through 3 different lenses.

The lens at the top clearly bends the light a lot more than the one below it. It is the most powerful. The one at the bottom is the least powerful.

There is an inverse relationship between the power of a lens and its focal length, i.e. more powerful lenses have a smaller focal length. The power of a lens (in Dioptres) is simply the reciprocal of its focal length in metres.

Power = 1 / focal length

So a lens with a focal length of 20cm would have  a power of 1 / 0.2 = 5 D