What do you think people mean when they talk about personal boundaries?

I’m hoping you can get concrete with this, and use real life examples. People throw around words like codependent and boundaries, and I even use those words, but I’m not sure I know what I’m talking about. It all seems rather vague in my mind.

What are boundaries? What are they good for? Is it bad when they are breached? What if you open them voluntarily? Does this place a burden on others? What do good boundaries look like? How should people behave?