A dominatrix is a person who is paid to act in a powerful or controlling way with someone else. They dress up in special clothes, like leather or a mask, to look scary and make it clear they are the person to listen to. They can make their 'partners' do things they wouldn't normally do, such as spank them or make them wear something silly. It is all just pretend and people do it to have a bit of fun and explore different types of relationships.