It means that what can one do when the heart is itself constrained; the earth is hard and the sky is far away.

This profound couplet speaks to the universal feeling of helplessness—a realization that despite our best efforts, some constraints are beyond our control. The poet suggests that the heart itself is captive, unable to break free. When the physical world (the earth) feels rigid and unforgiving, and the desired peace or destination (the sky) seems eternally out of reach, what recourse do we have? It is a beautiful lament on the limits of human will versus the vastness of fate.