In broad terms it means a pattern of controlling or threatening behaviour including some form of physical, emotional, financial or sexual abuse.
Things that could form that pattern of behaviour would include things like stopping someone from seeing their friends/family, depriving them of basic needs (food, electricity, etc.), monitoring their online activity, controlling when they can leave the house, controlling what they wear, controlling who they talk to, constant put-downs and belittling, humiliating them or forcing them to do humiliating things, controlling someone's access to money, stopping them from being able to work, threatening to reveal/publish embarrassing private information, etc.
In Giggs' case we know one accusation is that he threatened to send emails to her friends and employer about her sexual behaviour, for example.