I believe that what Thoreau meant by this is there are people who don't practice what they preach which is like unfortunate because a lot of people know what they are talking about and yet they don't truly understand it, and that is because they haven't experienced it. For instance talking about raising a child is easy but actually doing it so so complicated that you have to plan it out step by step. 

Experiencing things helps you give emphasis to what your speaking of, experiencing things makes you truly understand what you're learning and it makes you happy or sad and your able to interpret what they are learning and what's important. A Fulfilled life is a life that has been Lived through different experiences.