For example, when a husband physically abuses his wife that is a physical boundary being crossed.
When a friend calls you a loser that is an emotional boundary that has been violated. Part of taking care of yourself and being responsible for yourself, is letting people know when they are have hurt you or trespassed on your property.
Some people might be allowed to come into the yard, others on the porch, and the closest of friends come inside.
In sum, because you are responsible for you, you get to choose the people you want to share yourself with and chose the degree of intimacy you desire. We must let people know when they are trespassing on our property.
In our world around us, there are many examples of boundaries.