They are impromptu cafes in homes or literally in cafes, where people gather to talk about their experiences, feelings, fears, ideas, etc. about death. I've been hearing about them a lot lately and am really intrigued … given the power of the subject, I bet they can be quite amazing.
Here's an article about one held at the edge of Beachy Head – an infamous "suicide cliff" on the South Downs in East Sussex where I used to live. The article is by Claire Davies and was published in Aeon Magazine.