A sundial tells the time of day by the position of the Sun in the sky. In the narrowest sense of the word it consists of a flat plate (the dial) and a gnomon which casts a shadow onto the dial. As the sun moves across the sky, the shadow aligns with different hour-lines which are marked on the dial to indicate the time of day.
The style must be parallel to the axis of the Earth's rotation for the sundial to be accurate throughout the year. Unfortunately our sundial is not aligned however it sits in our garden and occasionally birds sit on it to sun.