For many generations humanity was unable to keep with time to the precision that is expected today. While we look at our iphones and our watches to keep time, there was an era where no time keeping was possible. Imagine how difficult it would be to make appointments in such a society.
The sundial is regarded as one of the first attempts to keep time. The earliest references to them come from about 1500BC from ancient Egyptian astronomy.
In general, sundials indicate the time by casting a shadow onto a fairly flat surface known as a dial face. A bit like a clock face.
The time is indicated where a shadow or light falls on the dial face, which is usually inscribed with hour lines. Although usually straight, these hour lines can also be curved if this is how the sundial is designed.