An opinion is something a person or a group of people think. "What is your opinion?" is like saying: "What do you think about it?"
The person's answer might start with: "In my opinion..........." or "I think........."
Sometimes companies try to find out what people think about something by asking many people their opinion. This is called an "opinion poll" and the people who do it are "pollsters". They might be trying to find out whether people think something is good or bad, such as electricity or exercise or Wikipedia, or whether one politician or beer or sport team is better than another.