The sign that is on the eastern horizon at the moment of birth is called the Rising Sign or the Ascendant. The Ascendant marks the difference in temperament, disposition, physical appearance and personality of individuals. It is the lens through which everything else in the chart is focused or looked through, thus coloring the way we look at everything. People see us through our rising sign and we see the world through our rising sign.
Your ascendant (also "rising sign") is the zodiac sign, that is ascending on the eastern horizon at the specific time and location of your birth. Its marked on the nathal chart as ASC or AS. If you know the precise time at which you were born, you can precisely calulate your ascendant.
Its accepted that if the ascendening sign is close to the Sun, its powers will be more noticeable. This is also true, if the Sun has weaker role in your Nathal chart (if you are born during the night, for example). Its also belived that the ascendant will have less impact if the moment of birth is at the end of the rise ( right before the next sign is rising).
Because the ascendant is directly tied to the birth time and location, it represents the childhood environment and conditions. This is why all other aspects of the nathal chart are determined from the rising sign.
Your rising sign represents the first characteristics other people see in you.