Trim, Stability and Control
The key concepts for aircraft control are trim, stability and controllability.
Trim is equilibrium of moments. Moments can be viewed as a rotational version of forces. Forces cause a body to translate, or move position, while moments cause a body to rotate, or change angle. Moments can arise when forces do not all act through the center of gravity of a body.
Consider a board balanced on top of a triangle shaped support, a seesaw if you like. If the board has a regular shape and uniform density, and you put the triangle support underneath the board halfway, it will balance horizontally. If the support is not directly under a mass, it will not balance. A moment is created by the offset between the support and the center of mass.