Abstinence was the abstaining of sex. It was a religious practice among the Cult of the Pah-wraiths. They claimed it was in line with the practices of ancient Bajorans, who better understood the value of denying worldly pleasures.
Cult members would take a vow of abstinence as part of their covenant with their master, Dukat. Exceptions were granted only when he believed husband and wife had demonstrated a true religious bond. (DS9: "Covenant")