I felt a Funeral, in my Brain, (line 1)
Either the speaker is imagining her own death, or something else has died inside her brain. Some critics think that she has "killed" and "buried" some painful or traumatic memory. The technical psychology term for this process is "repression." Others think that she is experiencing severe depression, which could be compared to the death of part of herself.
A Service, like a Drum– Kept beating–beating–till I thought My Mind was going numb– (lines 6-8)
The drum makes the funeral seem more like a tribal ritual than a sober occasion for reflection. The feeling of numbness comes after a lot of pain or sensation – much like death itself. Put another way, the numbness of her mind foreshadows death.
And then I heard them lift a Box And creak across my Soul (lines 9-10)
In this poem, the soul is not some rock-hard core of the self – it's a rickety old floor that could give out at any moment. Another fun fact about wood floors – they are made of a lot of small pieces rather than one large piece. Does this mean that the soul is constructed just like you might build a house?