If you play a sport you are familiar with the term "in bounds," which refers to the boundary, or limits of the playing field that the players must stay within. It is also the outer limits of any space.
A boundary is a border and it can be physical, such as a fence between two properties, or abstract, such as a moral boundary that society decides it is wrong to cross. If you have no sense of boundaries, you probably annoy people sometimes by getting too close to them or talking about inappropriate topics. Gustave Flaubert once said, “Earth has its boundaries, but human stupidity is limitless.” A boundary is basically what limits us.