Death, love and ambition are the three-fold themes of this poem. In the first four lines, the poet fears that he may die before he can achieve fame, or consummate his love. These lines show that he has rich and fertile imagination. This is brought about by the use of words like "glean'd," "garners," "full ripen'd grain." A harvest is the metaphor of fulfilment and Keats' mind yearns for it. His mind is full of thoughts and ideas but he has not reaped it all in the shape of poems. The reason is obvious. Death looms large over him and he fears that he may neither achieve fame nor consummate his love before death.
Death. More specifically, in those first four lines, Keats is afraid that he will die before he gets a chance to write all the ideas that are piled up in his brain. He's an ambitious artist, and he wants to achieve those ambitions before he dies.