An attractor is the curve characteristic of how the state of a particular system changes. It is produced by plotting two (or more) variables against each other; in this case the velocity and position along the path of the pendulum.
For a frictionless pendulum swinging in an ever repeating cycle the attractor is a circle (or ellipse) - becoming an energy losing spiral as friction is introduced.
Experiment with the pendulum's attractor
Why "attractor"?