Capital requirements, government policies, taxes, zoning, or control of resources are examples of barriers to entry. Breaking through them requires one to think carefully about the business that is being built.
Walls, doors, and windows are also barriers to entry. Getting to the the other side of these elements can result from tearing down a wall, opening a door, or simply opening a window.
The most challenging barriers to overcome are often those that we build around ourselves. Talking yourself out of a commitment, not trusting your instincts, or not be willing to take a chance can hold you back from taking the the next step in your career or life.
These barriers can also be the easiest to overcome. Sometimes a small shift in perspective is just enough to do the trick.