The language of movement is designed to allow us to describe how the body moves through space.
In exercise it allows us to communicate with other movement professionals so we can describe movement in a common language. This helps avoid confusion, as we are all speaking with the same frame of reference.
The basis of this frame of reference is the Anatomical position, famously used by Leonardo Davinci
in his drawing The Vitruvian Man.
The anatomical position is slightly different to the Vitruvian Man in that the arms are closer by the side of the body, but everything else is essentially the same.
This position, standing straight, arms by the side, palms forward is the position we base all movement description from this reference point.
Is the when a joint bends to decrease the angle between two bones.
Example: Elbow flexion, or doing a bicep curl creates a flexion movement at the elbow joint where the humerus and ulna join.
In the picture below the starting position is essentially in the anatomical position. The only joint moving in a bicep curl is the elbow, specifically called elbow flexion on the up phase.
Flexion is also used commonly at the Knee, such as in a lying leg curl, which uses the hamstrings to bend, or flex the knee joint.