In "When I Have Fears," Keats is afraid he will die before he can fully experience love and before he can fully develop as a writer. In line 4, the "full ripen'd grain" represents the fully ripe, artistically developed ideas in his poetry. He also hopes to be prolific, noting his wish to have piles of books before he dies. (The "charactry" are characters or letters of the alphabet).
The "magic hand of chance" is his writing hand. He fears that he might die before he can develop his imagination enough to adequately "trace" the "huge cloudy symbols of a high romance." Here, the speaker (Keats himself) is blatantly saying that nature is symbolic of romantic ideas and nature is one of the muses of the poet; that is to say the poet's contemplation of nature can provoke the imagination which can make his poetry come alive.
In the final four lines, he considers if he does not experience love nor develop himself as a poet, that it would be as if this love and fame will sink into the water while he stands hopeless and alone on the shore.