This is a side view of a concave mirror. The black horizontal line is the principal axis. You can move the candle alongside and observe the mirror image (now it is an upside down reflection). The first red dot on the axis is the focus (the reflection will flip, when the candle is there), while the second one is the centre of the curve (at this point the sizes of the two candles are the same). The closer you are to the mirror the bigger is the reflection. You can try it with a shiny spoon, too.