How does silica get into the reef aquarium and what does it do?

Silica or silicon dioxide is actually bound silicon and oxygen, also silicon over all, is the second most abundant element on earth.

Okay how does this hard substance get brocken down and made water soluble?

Silica is only slightly soluble in water, normally!
The variations of the make up of the silica-containing minerals that make contact with water and the pH of the water are the primary factors controlling both the solubility and the form of silica in the resulting solution.
Silicates are bound with a multitude of minerals and much more, thus making it quite solid, you take away the bind in one way or another and its water soluble!

In aquariums, with the cycle, once maturity is achieved, the cycles bacteria produce abundant acids to sort of live amongst, one being nitric acid!
Silica is profoundly affected by varying PH and as we all know, acids are extremely low PH, these are how silicates are made water soluble in the most abundant ways.

If you buy sands from landscape supplies or places like bunnings that are high in solid silicates, once the aquariums bacteria is living in that sand with it, their life style starts making it even more soluble.

That's how non abundant silica becomes a problem and inspires pests like diatoms using it to make their husk/outer casing, to name just one!