It means they had sex.

The 'Do Bits Society' is probably the worst name for a group of people who have been sexually active in the whole entire world. It's childish, it's braggy, it's annoying, and we're not just bitter and jealous at all. Fair enough if you don't want to talk about it lads, but running around and asking whether other people have "done bits" when you know they haven't is juvenile and rude and we're sick of hearing about it, to be honest. But we're not going to stop reporting on new members, so whatever. In tonight's episode of Love Island, another couple will join the Do Bits Society (DBS now, for short) after a romantic night in the Hideaway. And no, before you ask - it's not Dr Alex and the blonde woman whose name we can't remember. The Hideaway would never recover. It is in fact, in the least shocking turn of events, Josh and Kaz who do the deed. Or at least do some form of the deed which some people wouldn't class as the deed but it's still a pretty big deal so fair play. Later on tonight, Dr Alex will get a text and declare that the Hideaway is open. Josh and Kaz are the ones who hop on in, only delighted that they'll get to spend a bit of time together. To do bits. Nice. The next morning, the pair emerge to have a debrief with members of their respective genders in which Kaz confirms that they did indeed do bits and that she thinks she's falling for Josh. Aww. Wonder what Georgia'll have to say about that.