Basically this is a poem of a man who chooses to live a solitary life somewhere, connected to nature and disconnected from other humans. At first, he is full of ambition and strength:
At first he was out with the dawn
whether it yellowed bright as wood-columbine
or was only a fuzzed moth in a flannel of storm
He is up early, working and learning to live on the land. Gradually, however, the isolation begins to manifest itself in unrealistic fear.
When he tried his eyes on the lake ospreys
would fall like valkyries
choosing the cut-throat
He took then to waiting
till the night smoke rose from the boil of the sunset
As his fear of things that do not actually exist develop, he will only go out at night. He starts to believe that things in nature are out to kill him. Until, finally, the fear becomes so intense that he eventually loses his mind.
And now he could only
bar himself in and wait
for the great flint to come singing into his heart
At the end of the poem, the man no longer goes out at all. We assume this means he will die, as no one is nearby to take care of nor rescue him.