A ram-raider is the person who steals by driving a car or van straight into a shop, smashing the window (or even the front wall), then jumping out, grabbing goods — cigarettes, phones, jewellery — and racing off. The crime is British newspaper English more than classroom English, especially from the 1980s and 1990s, when this smash-and-grab method became a story people recognised. Though actually the sense line names the crime; the word with -er is the thief. The crime itself is ram-raiding. “Ram” here is the hard hit: you ram something when you drive into it with force. A ram-raider is not any robber. You would not use it for a pickpocket or a quiet burglar. It sounds like police-and-tabloid talk: dramatic, a little dated, still clear if the shop was literally hit with a vehicle.