It’s been in practice by our elders for a long time to look out for omen. When two Brahmins come together while we go out, it is considered a good omen.
There is something called nimittam; when a bell chimes before we proceed to do a work, it is considered good sign and the work shall be successful. A neran is one that happens at that time for e.g., when someone unintentionally utters negative words at the time we proceed to do something, the work we intend to do may not have the desired result.
Dreams also have positive and negative results accordingly. Ramayana is filled with many omens. Astrology also deals with such omens. For details on scientific explanation, please click this link