The lines AP and BP are tangents to the circle, each touching the circle only at one point. You can click and drag the points A and B, which has the effect of changing the size of the circle.
Dragging point P allows you to investigate the tangents from P to the circle. Look carefully, because there are two important theorems (rules) to learn.

You can choose between the two theorems by clicking the two small circles at the bottom of the page. If you want to see the description of each theorem, then click the "Show Theorem" box, but try not to do this until you have tried to work out the theorems for yourself. Enjoy yourself while you practise and learn these theorems by name.