Having a private account means that only you and your followers can see what you post. People will have to request to follow you to see what you post and you can approve or deny their request.
When you create an account, your profile will be public by default. To make your account private, go to your settings and change your account privacy settings in the ‘Privacy’ tab. Anyone who was following you before you made your account private will still be able to see tour posts. If you don’t want someone to see what you are posting you can block them.
Remember, one of your approved followers can still screenshot and share the things you upload, so be careful about what you post.