Have you ever seen a circle?
Most of us would say that we have, but we actually haven't. A circle is defined as "a plane curve everywhere equidistant from a given fixed point." In its purest sense, a circle cannot be drawn--it is a one dimensional curve in a two dimensional space. It can't exist in physical space at all. It's an idea, a concept.
You may see pictures that represent circles, but the only circle that I know of is the idea of one that I have in my head (and that I assume is also in yours).
This is true for many more things than circles. Honesty, for example, or truth. These are things that do not exist except as mental concepts, and, except for very simple things like circles, our mental concepts are often unstable and very seldom in complete agreement with each other.
Here's another thing that only exists as a concept--the self.
Though we all have a strong sense of self, and our minds insist that we must have an identity, no one has ever been able to find a self. You can test this. See how what you think about yourself changes from day to day and moment to moment. Your very memories can changes--notice how often we disagree about what happened. All we seem to be is a changing physical location that experiences a stream of thoughts that is constantly changing, too.
We spend so much time trying to pump up and protect this sense of self, because it seems to be so fundamental to what we are. And yet this is what we are trying to protect? An uncontrollable thought stream?
It's only in those rare moments when we are without thought that we see the very real possibility of living without boundaries. We see that the only boundaries that exist are the ones that our thoughts create. And when we begin to see through those boundaries, we see freedom itself. We stop fighting about concepts, and we gradually see that our lives are almost nothing but concepts.
And life becomes easier (it begins to life itself) because there is less and less in the way.