Just as a refresher, every birth chart has 12 houses that govern particular themes in your life. The first house, for example, deals with identity and beginnings, while the 12th house deals with releasing and endings.
Your first house, like your rising (or ascendant) sign, is determined by where the sun was rising over the eastern horizon the moment you were born. From there, other planetary placements are determined.
The houses that the various planets fall into are thought to influence your personality. For example, if Mercury, the planet of communication, is in someone’s fifth house (which is all about expression and theatrics), they might be particularly outgoing and boisterous.