You can follow any Public account and anyone can follow you if you don't have a Private account.
Followers
Your followers can see all the Photos and Videos you post on your profile. To prevent a follower or anyone from seeing your posts, you can block their account. People aren't notified when you block them. Learn more about blocking people.
Mates
If you follow back your follower or someone you follow, follows you back, you become mates. In short, people that follow each other are listed as mates. They will still be listed in your followers and following lists. To remove someone from your mates, you can simply unfollow that account.