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..........."
Sometimes companies try to find out what people think about something by asking a lot of people their opinion. This is called an "opinion poll". They might be trying to find out whether they think the government is good or bad or whether enough is being done about global warming.