The fundamental theme of this poem is regarding the significance of succumbing to death, for after having a full life everyone must fearlessly face the end. In addition, the poem emphasizes that one should not fight against the arrival of death in any of its forms.
Fear no more the heat o' the sun' is quite clear, it is death.
The lines of this poem/song are quite simple, especially for Shakespeare, and convey quite clearly the speakers' interest, death, and the escape it provides from life.
