There are two shapes on the app. The left shape is the original shape with where the possible line of symmetry falls on the triangle. The right shape is the exact same shape, but it folds the shape across the possible line of symmetry.
The red point on the line allows you to drag the line around the shape. Drag the red point in a circle around the shape to check for all possible lines of symmetry.
After you figure out how many lines of symmetry there are, put your answer in the top right yellow box. When you push "Enter", you will get a pop-up box that tells you whether you are correct. If you are incorrect, it will give you a hint to help you try again.
Once you get it correct, use the slider in the top left corner to go to the next shape. Try all 14 shapes!