Boundaries keep the good in and the bad out. For example, your skin is your first boundary, it keeps the bones, blood, etc in and germs and foreign objects out.
Boundaries help you know what is and what isn't your responsibility.
Sometimes we have bad on the inside and good on the outside. If boundaries are the fence, then we need a gate in the fence. Some people are hurt by a boyfriend, parent, or other trusted person, so they say, "I'll never let someone that close again", and in that way make it impossible for someone to love or care for them.
Boundaries are not walls.
Boundaries are anything that help differentiate you from someone else, or shows where you begin and end.
I will add to this subject next time.