A gold standard is when gold is used as the basis for a monetary system. It’s also any excellent example of something, like how Olympians are the gold standard for athletes.
Originally, a gold standard was literal — it referred to gold being the official measure of money. Money was valued in reference to the gold standard. Gold is no longer used in this way, though it's still valuable, so the term has taken on a looser meaning. The gold standard of something is simply a great or excellent example. A gold standard is the best of the best.