This word is usually used with out. For example, if you are qualified to participate some benifit plan in your company, you may opt out.

Recently, my eight-year-old was enrolled by the gifted and talented programm in our school district. In the notice I received, it clearly stated that we can opt out if we are not interested in the program.

By law, all the children over 6 in my state should go to school. However, parents can opt out their kids. That means the parents/guardians can keep the children home and teach themselves. This is not kidding. Some parents do so because they don't rust public school systems.