They are "photochromic", meaning they react UV light. A chemical in the plastic gains an electron when struck by UV radiation and its shape "twists" into a different shape. That new shape blocks visible light. The more UV, the more the lens material darkens. Remove the U.V. and the material releases that electron and the shape twists back to clear.

One last thing, that chemical in the plastic is also reactive to temperature. The hotter it is, the less likely the chemical is to absorb that electron. So if you have a very sunny but very HOT day, your lenses may not get very dark. Conversely, if you have a very cold day with hardly any sunshine, your lenses might go very dark.