The rising sign
The sign on the horizon when you were born. The face at the door.
The rising sign is the sign coming up over the eastern horizon at the moment of birth. Because the Earth turns once in 24 hours, it changes roughly every two hours — which is why the birth time matters.
Not a mask, a surface
The rising sign gets called a "mask", which is unfair. A mask is fake; this is not. It is the surface where you meet the world — how you walk, how you handle a stranger, what you do when you enter a room.
Why it matters so much
The rising sign also marks the start of the first house — meaning it sets the whole house layout. Change the rising sign and you change which area of life each planet lands in. That is why it is the single most information-heavy point in a chart.
Your own rising sign, and what it means, is below.