In my opinion, this poem means that, although time passes, we keep what is important in our hearts.  These things that we retain are much more important than the physical things that we lose.

We can see this in how the physical things in this poem have been lost.  Specifically, the wooden steps are no longer there.  They are off somewhere, waiting to be burned.

Even though the steps are no longer there, the memories that are associated with them are there.  They linger in the speaker's heart and they make her happy even after time passes and physical things are gone.