Emily Dickinson carefully constructed "I heard a Fly buzz--when I died--" to have perfect iambic meter. The word 'meter' describes the pattern of stressed and unstressed syllables in a poem. This particular poem is written in iambic meter, which means Dickinson wrote each line to be in a pattern of an unstressed syllable followed by a stressed syllable (like Duh-DUM). Here is an example for you:
"I heard | a Fly | buzz – when | I died – "
Even the structure of the stanzas is carefully organized to support Dickinson's iambic meter; she divided the poem into four stanzas, and each of those stanzas have the same division of syllables per line. The first and third lines have eight syllables each (known as 'iambic tetrameter', because 'tetra' means four--so it has four iambs). The second and fourth lines of each stanza have six syllables each (iambic trimeter).