Its a bit hard for me to explain but headspace is a bdsm dom/sub play in general is when you let yourself go and getting “hight in your mind” like getting too much into it and -almost- loosing yourself. The most know one is little (ddlg) age regressing, but there is more, like domspace, subspace, into pet play etc.. the drop is when you falling like from your hight too much to fast or not in the good way, no aftercare or simething gone wrong in the play, there can be a drop in all kind of headspace

Here my broken explanation of what i understood by reading a lot of dirty fic lool, hope it help
(You can also read ao3 fic with “headspace”)