Parkour is a means to fully utilize one's body in weaving through obstacles presented by the environment.
This form of physical activity requires discipline and a certain degree of self-awareness.
A traceur needs regular physical training and conditioning, aside from breaking down mental barriers that hinder the body's performance.
For a beginner, it's all about being able to execute a move and being aware of what his/her body can do, and if the obstacle is suitable for the intended stunt. During training sessions and actual runs, careful assessment of one's capabilities will prevent unnecessary injury.