Sail trim consists of shaping the sails appropriately to the point of sail (or relative wind direction to your boat).
To understand sail trim, we need to know the anatomy of the sail first. This detailed guide will teach you everything using clear diagrams.
More on sail anatomy
Expert sailors know how to get more out of their boat with good sail trim. When you trim your sails, you get them to the optimal shape to provide the most driving force for the wind conditions and point of sail. It's a skill you can learn by understanding the basics of how the sails work and how the sail controls affect them.
Points of sail
Every direction of sailing relative to the wind has a name, and the meaning of that main is a critical piece of understanding you'll need when you're learning to sail. The main points of sail from straight upwind are beating (or "close hauled"), reaching (close, beam, and broad), and running. There is also a no-sail zone straight upwind, though this is not generally viewed as a point of sail since you can't sail there.