A geas is a taboo against something that is placed upon you (mostly voluntary, but can also be forced) that, as long as you observe it, gives you something. Violating a geas often gives very harsh penalties (in mythology, often death).
example (I don't know how they work in 5th edition, so I'm just putting something here): As long as you don't eat meat, you gain 2 extra dice on your drain tests. Should you eat meat, you get a penalty of 2 dice until you do penance.
word of warning: don't take too many of those, as they might become mutually exclusive (observing one violates another). The mythic Irish hero Cúchulainn had a geis to never eat dog meat, and he was also bound by a geis to eat any food offered to him by a woman. When a hag offered him dog meat, he had no way to emerge from the situation unscathed; this led to his death.
