the explosive lens is a "lens" in the sense that it can focus shock waves. To compress a sphere into a tiny ball, you have to have continuous equal pressure over the whole surface or else it will deform and splash into a complicated shape.

You cover the plutonium sphere with an shell of explosives (or a shell of some dense metal that is then covered with a shell of explosives.) If you start to detonate the explosive shell using a detonator at one location, the shock wave will hit the plutonium directly under that location first, starting a dimple. The plutonium on the other side isn't being compressed because the explosive there hasn't started to explode because the shock wave hasn't arrived yet.

So you use a bunch of detonators all over the surface. That's better, but it still creates an uneven pattern of pressure -- now maybe you get 20 or 60 dimples forming symmetrically but it's still not going to result in a compressed ball. There will always be locations on the surface of the plutonium where the shock wave is pressing the material sideways instead of inward.

The explosive lens uses two materials with two different speeds at which the shock wave can travel. The shock wave directly under the detonator is going through the slow stuff but the shock wave spreading sideways from the detonator is going through the fast stuff. If you shape these in the right pattern, using curved interfaces, the effect is just like light passing through a curved lens. The shock waves are bent into a pattern that is almost equal pressure everywhere at the surface of the plutonium.