This quote is saying that in order to make progress in life, you must take risks and be brave enough to let go of your comfort zone. To reach new heights, you must first be willing to let go of what is familiar and try something new, even if it involves taking a risk and not knowing what the outcome will be. The metaphor of the shoreline represents safety and familiarity, and the new horizons represent potentially unknown or unexplored realms of life experience.