It means that borderlines can, under emotional / environmental stress, have paranoid ideas - i.e. believe weird things to be true that aren't. For example, someone might believe their girlfriend or boyfriend wants to kill them when there is no evidence for that; or that the police are monitoring them without evidence. The "paranoid" aspect implies something that is out of contact with reality, but only partially. In that way it's slightly related to paranoid schizophrenia, which is a much bigger, complete break with reality.

The dissociative symptoms describe how a borderline can divide up their emotional/perceptual experience so they don't experience the world as one person. For example, someone can create a "child self" that contains the pain of their past abuse. Or, when meeting an attractive woman, a borderline man might be threatened by the emotional closeness and dissociate emotionally - i.e. cut off his feelings and seem emotionally deadened and uninterested. Dissociation is a clever defense of splitting the mind in pieces in terms of how one experiences it, usually to avoid expected harm.