I stepped from plank to plank:

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                    5
  Would be my final inch,—   
This gave me that precarious gait   
  Some call experience.


Analysis:

The speaker, perhaps, is walking on a boardwalk near the beach. She feels the stars about her and the sea beneath. "The stars above" can signify one's dreams to aspire to the greatest while the line "About my feet the sea" shows the impending doom of a wrong step in life which has made the speaker to walk cautiously. She moves cautiously not want to fall. 
In the second stanza, the metaphor becomes clearer. This is the slippery boardwalk of life. She doesn't know whether her next step will be her last (that is, who knows when our life will unexpectedly end?) This knowledge that life is finite makes her walk carefully.
The unpredictable nature of life makes it a challenge to foresee the right choice we've made at a given time and place.