It means that you shouldn't let what happened in the past bother you, or worry about what the future holds for you. Because neither are as important as the strength of character you have that will help you overcome both your past demons and future worries.
Or at least that's how I interpret that.
I would like to add a bit more about this:
A sea shell found at the shore when is brought to an ear, makes us feel that the whole of the ocean is within it; it roars in such a way: the same is with we humans. As we find the whole of the ocean as if within the shell: the whole of the ocean the shell is found at, we humans have within us the power of the whole of the universe, we live in and perhaps it's even more.
This is perhaps an other way of interpreting the same statement in terms of capabilities.