Abigail, Tituba, and the girls were dancing around a fire that was supporting a boiling pot.  Most of them were casting harmless "spells" they had learned from Tituba, but Abigail's spell was different.  She drank a charm that was supposed to kill Elizabeth Proctor, John's wife.  She assumed that if Elizabeth died, John would marry her because she had had an affair with him.

It is important to understand also that these girls willingly participate because Tituba is creating a LOVE spell for the girls who have their eye on someone in particular.  Abigail has her eye on John Proctor.

In addition to Rene's answer, it must be understood that the girls willingly participate in this activity. None were coerced.  They are intrigued by the mysticism of Tituba's Caribbean occultism; they feel oppressed by the stricture of their Purtian upbringing. 

Furthermore, Purtians believed that behavior such as this would surely paint both the participants and their family as doomed, and not among the "elect" (those pre-destined for salvation.)  The girls' illicit activities in the woods, therefore, had ramifications not only for their own souls, but the salvation of their family members.  This is one of the many reasons Abigail becomes so intent on keeping their "sin" a secret. 

Tituba takes the girls out into the woods, and they have a ceremony of sorts. Abigail drinks blood as part of a spell. The girls were discovered by Reverend Parris, who had seen Tituba “waving her arms over the fire” and had heard “a screeching and gibberish coming from her mouth.” Parris suspects the girls had been practicing witchcraft-which is punishable by death.

Betty falls into a trance after the discovery, and the evening activities begin the hysteria of the witch hunts.