In level flight, and with no control input, a properly trimmed airplane is in perfect balance respect to its center of gravity (CG).
All the forces and moments that are acting on the structure and surfaces become equal and opposite, just canceling each other.
It is as if the whole aircraft were hanging from that single point (CG) in a horizontal and steady position.

For models of normal configuration, when the pilot deflects any of the control surfaces, the reaction of the airstream over the control surface creates a force.
That force is applied far from the CG (where the control surface is located on purpose), which creates a moment or torsion force around the CG, and, as a result, that previous perfect balance is altered.

By convention, three imaginary lines cross the CG, which are called axis:

Longitudinal Axis: Extends lengthwise through the fuselage, from nose to tail. Rotation around the longitudinal axis is called roll and it is produced by the deflection of the ailerons located at the trailing edges of the wings.

Lateral Axis: Extends crosswise, wing tip to wing tip. Rotation around the lateral axis is called pitch and it is produced by the deflection of the elevator at the trailing edge of the horizontal tail.

Vertical Axis: Passes vertically through the center of gravity. Rotation around the vertical axis is called yaw and it is produced by the deflection of the rudder located at the trailing edge of the vertical tail.

When deflection of only one of the mentioned control surfaces generates rotation around the axis that surface controls plus rotation around one or two of the other axis, we say that the specific model shows control coupling (yaw-pitch coupling, yaw-roll coupling, etc.)

Coupling is a common condition, which is more pronounced in self-leveling models, like trainers and 3-channel gliders.
For precision aerobatics, the condition is not desirable, and is frequently eliminated by radio mixing and/or careful trimming.