You can move the points A, B and C around by clicking them and dragging them with your mouse.
When you start you are looking at the first theorem. As you move the points, notice that the angle B never changes. This is a special rule (theorem) which you should learn. You'll see what the rule is called when you click the "Show Theorem" button.

If you select "Theorem 2", the situation is a little more complicated. Look at the relationship between the angle B and the angle at the centre. This shows another theorem. Try to work out the rule before you click the "Show Theorem" button. Enjoy yourself while you practise and learn these theorems by name.