It goes back to when there were bimetallic currencies, and governments would establish official rates for the value of silver vs gold.
Under the Spanish Empire [I think] it was about 16 to one, and we (the US) inherited the basics of that system except we split our Peso-- the Dollar-- into hundredths instead of eighths & fractions of eighths.
Not long after, I believe silver was allowed to float, and float it did-- or sank. The ratio rose on the market, gold was worth lots more than 16 to one.
It keeps changing, recently went over 100 to 1, has since come down again.
Why does it matter? It might not, but some feel there is a "proper" ratio that it will trend back to. I don't agree.
What does happen, though, is that when the ratio gets much higher or lower than a perceived norm, people will exchange the "overvalued" metal for the lesser, and then vice-versa if it hits the other extreme. Every sale, of course, involves a transaction fee, so a perpetual motion machine, it's not.
You could pick any two commodities and do this. But gold and silver are magic.