Do you have a criminal conviction in your past?
Canada, like most countries, has laws excluding visitors with criminal convictions in their past.
This would include things like: theft, assault, manslaughter, dangerous driving, driving while under the influence of drugs or alcohol, or possession of or trafficking in drugs or other controlled substances — and more. Some are major and some minor. Here are some links that can help you understand how you can still be admissible to Canada: