Imagine you are trying to get somewhere, to go visit a friend, but along your journey you run into wall after wall that barricades your path. It would make getting to that friend much more difficult. Instead imagine that same journey, and at every obstacle, like large rivers, difficult ravines, etc., there was a bridge there for you to cross. That would make getting to your friend much easier. If you take that idea and put it into symbolic form, people should be doing all that they can to build bridges to friendships, relationships, happiness, prosperity and freedom. If someone's goal is to achieve their dream of becoming a writer, for example, a way to build bridges for that person is to encourage them, give them outlets for their writing, and find chances for them to practice. That would be building bridges. If instead you were to discourage them, criticize their work too much, and stunt their dreams, then that would be building walls.
This thought, of building bridges, not walls, can be applied to almost any situation. If you are trying to get straight A's, building a bridge to get there would be to study hard, pay attention, get help when needed, and form bonds with others who can help. If you were to build walls instead, you might let your fear of talking to the teacher get in the way of asking help, or your shyness get in the way of forming study groups, and your not wanting to study get in the way of doing well. Those would be walls that you have built that impedes your success.
I hope that those examples help to explain the quote a bit; good luck!