Light from the candle on the left passes through the convex lens and forms an image on the screen on the right. The farther the candle is from the lens, the smaller the image, and the closer it is, the larger the image. The shape of the candle appears sharp only when the screen is exactly where the image forms.
When the candle is farther than twice the focal length (2F), an inverted real image smaller than the object forms. At 2F the image is the same size as the object, and between F and 2F it is an inverted real image larger than the object. Place the candle at the focus (F), and the light travels in parallel without forming an image. Move it inside the focus, and the image becomes an upright virtual image that does not appear on the screen. This is what you see when you look through a magnifying glass.
The view shows the same three rays you would draw to find where the image forms: the ray that travels parallel to the axis and then passes through the focus, the ray that goes straight through the center of the lens, and the ray that passes through the focus and then leaves parallel to the axis. Where they meet is where the image forms.
You can drag the candle and the screen left and right. Drag an empty area to rotate the view, and use the mouse wheel to zoom in and out. You can move a panel by dragging its header. Double-click a header to switch to a layout that shows one panel at a time at the top right.