This sugya discusses the issur of Nichush - i.e. interpreting an event or action as an omen or "sign" - which is a Mitzvas Lo Sa'aseh mentioned in Vayikra (19:26).
In Parshas Chayei Sarah, Eliezer devises a plan to find a suitable wife for Yitzchok based on a "sign" - i.e. the girl who will offer water for him and his camels. The Rishonim debate as to whether or not this falls into the issur of Nichush. Based on this machlokes Rishonim, we discuss various "Halacha L'maaseh" scenarios. Some examples include: eating simanim on Rosh Hashana, the custom of not getting married towards the end of the month, etc.