Your Rising sign (also called Ascendant) is the zodiac sign that was rising on the eastern horizon at your exact time of birth. Because Earth rotates completely in 24 hours, each sign rises for approximately 2 hours.
This is why knowing your exact birth time is crucial for an accurate birth chart. Being born even 10 minutes apart can change your Rising sign, which affects your entire house system.
Your Rising sign represents how others perceive you and how you approach new situations - it's literally your "mask" to the world.