I often read in various forums that a good upgrade to make on a bike is the wheels. I'd like more information on what to look for in a set of new wheels. What are the measurements I need to understand, how are wheels sized, how do I know if a wheel set will fit a certain bike?

For instance, I have a Trek 920 (2017). It is specced as "Bontrager Duster Elite tubeless ready, 15mm front 142x12 rear." If I were to upgrade this, what specifications do I need to pay attention to? How can I tell if a particular wheel is an upgrade, would fit my bike, is appropriate for my type of riding, etc.?

I'd appreciate any pointers to blogs, websites, etc. that explain wheels, etc.