I stepped from plank to plank
So slow and cautiously;
The stars about my head I felt,
About my feet the sea.
I knew not but the next
Would be my final inch,–
This gave me that precarious gait
Some call experience.
I suppose the speaker is walking along a hanging bridge made up of planks. His movement is very slow and cautious because the way is unsafe and dangerous. The speaker feels like setting out his way across the sea at nighttime when stars are above his head. So he cannot see his way ahead very clearly and he might drown into the sea any time if each of his step is not careful enough. Although the setting seems like threatening, the speaker has his dream which is signified by the starry night above his head. The speaker does not know when will be his final step. But before his final step, the manner of walking dangerously along the planks gives the speaker the valuable experience.