You are visiting your grandpa and he asks for a hug but you REALLY don't want to. Do you have to do it? What can you say?
"No, thank you." You do not have to do anything with your body that makes you uncomfortable (except sometimes at the doctor or dentist).
What is a boundary?
A boundary is a limit. There are physical and emotional boundaries. Everyone gets to decide for themselves what does and does not feel okay.
Your friend asks for privacy to change into his swimsuit in the bathroom. You need to get something while he is in there. Can you just walk in and get it even though he closed the door or should you wait?
You should wait. It is important to respect his boundaries and privacy.