Presumably, it's a physical manifestation of the song's lyrics, where Twigs plays a woman trapped in a web of her own making, obsessing over a man who "only want(s) me in other spaces". The backing sounds like a close-up, slowed-down recording of a spider spinning silk, which only adds to the video's sinister aesthetic.
Oh, and if you're thinking "yeah, but everything's CGI nowadays", take a look the rope marks in this freeze frame.
Gruesome - but as The Verge points out:
"FKA Twigs began her career as a dancer, and her physical work and visuals frequently trend towards dark, sensual material that challenges perceptions of dominance. Her video for Papi Pacify, off 2013's EP2, featured Twigs wrapped in a man's embrace, unable to move, while his hands pried open her mouth. Her recent visual for Video Girl had her dancing seductively around a man about to be executed. Barnett doesn't seem to be interested in moving towards a feel-good aesthetic."
FKA Twigs plays the Brits launch party tomorrow night. I imagine ITV are crapping themselves right now.