If you abstain from something, you restrain yourself from consuming it. People usually abstain from things that are considered vices — like drinking alcohol or eating chocolate.
Roots of the word abstain are from the 14th-century French, "to withhold oneself," and the word often refers to people who abstain or keep themselves from drinking liquor. The noun form abstinence also pops up often in reference to abstinence programs that urge teens to abstain from premarital sex. Abstain can also mean to withhold a vote, and sometimes a difficult decision is held up when government representatives abstain from voting one way or another.