What is a Guardian?
When you write a Will you can use that document to specify who you want to look after your kids. Obviously if your wife/husband is still alive and currently looking after them, they will continue to do so. But if there is no one like that, the person you name will have the opportunity. They are not legally obliged to do it - so it would be a waste of time to nominate your worst enemy to look after your teenagers.
In other situations Guardian simply means someone who looks after someone who is not capable or old enough to do it themselves.