Your rising sign — the zodiac sign on the horizon at your exact birth time — is supposedly what colors your first impression on strangers. Is there anything to that claim? Partly, and the reason is less mystical than it sounds.
Rising sign is a function of birth time and location, full stop — it has nothing to do with personality traits by any mechanism astrology can point to. But people's sense of "first impression energy" is real, and it's shaped by things that do correlate loosely with time of day: energy levels, routines, even lighting and context around when you tend to meet new people.
So the honest answer is: rising sign itself explains nothing directly, but the folk wisdom around it survives because people who present a certain way at first meeting keep getting typed into the same handful of "rising sign vibes," which then reinforces the belief. It's a real pattern with the wrong explanation attached to it — which describes a lot of astrology, if we're being straight with you.