Mike Caro coins this same phrase in his portion of Doyle Brunson's Power Poker. Not sure why this phrase keeps popping up in the poker world.

Thoureau was basically saying that the majority of men don't live out their dreams. They just go about living their ordinary day to day lives and only in fleeting moments do they attain some sort of personal accomplishment or attainment of that dream. He is trying to get the reader to seize the moment.