As it says on the label: behaviour that isn't stable. They're impulsive, they indulge in something (e.g. drinking, drugs, sex, food), they act without thinking about the consequences. They might not care about the consequences. Their emotions might be all over the place. They might fixate on something and how to repair that something. For example, if they went through a breakup they might fixate on that and they might have a lot of sex with random people in order to sort of compensate for the loss. They might also binge eat/drink. The things they do are entirely self-destructive. They might do a lot of what they do because they convince themselves that that's what's necessary to stop themselves feeling whatever they might be feeling. I've experienced "unstable behaviour" a few times. It is as screwed up as it sounds.