What this house holds
The first house starts at the eastern horizon of your chart. Whatever sign was rising there at the minute you were born is called your rising sign, or Ascendant.
I think of it as the doorway of the chart. It shows how you come into a room, how much you hold back or put forward, and the part of you people meet first. Your Sun sign is closer to who you are underneath. The first house is how that self steps out.
How it was read before
The older books gave this house to Aries and to Mars, because Aries is the first sign and this is the first house. They saw it as the seat of the personality and of how active or restrained a person would be.
They also read it for whole countries. When a chart was drawn for a nation, the first house stood for its ordinary people and the general mood of the place.
Sit with this
Your rising sign is the part of you that greets the world. It can be very different from how you feel inside, and that is fine.
Notice when you lead with it, and when you hide behind it. You get to choose how you show up today.