I believe the underlined phrase should read: "we are limited by what we bring to it." "It" means the task at hand, which in the text you gave sounds like the task is life in general.
It appears that this paragraph is saying your life is what you make it. It can be as exciting or interesting as you'd like, if you put the effort into it. What you "bring" with you refers to any special talents, education or experiences you might have that can shape your life. For example, do you have a very wild imagination? That might help you to invent or create a life-saving medicine one day, because you refuse to believe it when someone else tells you "it can't be done."
Sometimes instead of "what we bring to it" we say "what we bring to the table;" the meaning is the same. What special skills or experience do you have? What are you willing to do?
Suppose you have interviewed for a teaching job, and the administrators are trying to decide between you and another candidate. One person might ask:
"They're both highly qualified. What exactly does Juliana bring to the table?"
The person that interviewed you might then reply:
"She has an advanced degree, and has spent two years working in England as a nanny, so not only does she speak English very well, she also has experience working with children."
Student or Learner