C19H28O2

What is this molecule and why is it important in exercise?  Or, better yet, why is it important to know it's effects?

This is testosterone.

Testosterone is one of the most critical hormones in muscular development and often is overlooked for other things.

My focus with testosterone is not in the anabolic (growth) nature that it provides, but on the effect that it has upon the nervous system.  When bound to a nerve in some way, it increases the Action Potential frequency (action potential is essentially an impulse down the nerve).  When you increase the firing frequency, you typically increase the muscle recruited to perform a task.

When you increase the muscle used to do something, you ultimately increase it's size, strength, and it's power.  It's basically a cascade of events that lead into the ultimate desired goal.  In this case, starting with a hormone and ending with the targeted goal.

Increasing free testosterone is very difficult, more for women than in men, but nevertheless difficult.  A simple way of going about it is to commit to a long-term training plan that over time will increase it some, but depending on the person it could be in large magnitude or in small.

So ultimately, by increasing free testosterone in your body, you'll have the training effect that you likely desire.  Which usually is increased muscle size, strength, and speed, improved fat-burning capabilities, and ultimately a body that you can be proud of.