What is the poem 'I heard a fly buzz, when I died' by Emily Dickinson about?

That's such an important poem to me! I still don't know whether it comforts me more than it scares me. Scares me, I think. It's a dark, dark poem. If I'd make a recommendation, though, it'd be to read it without knowing anything about it. And then spend a few days thinking about it and maybe re-read it again.

I'll happily tell you what it means to me, of course, and I also know what students of poetry think of it (I caught some but not all of that and something I only half-sensed was explained to me when I read about it). It's about death, by the way, but I'm not giving away any secrets there. For some reason, many of my favourite poems are about death.