It’s what was known as “bodyline”, or “fast leg theory”. In essence, bowling in a way that the ball is bowled at speeds potentially in excess of 140 km/h aimed at the batsman’s upper body or head. It makes it very hard to hit the ball, and if you do hit it, it drastically reduces the types of shots you can play and increases the chance that you only get an edge on the ball. So setting a field with a large number of fielders on the leg side (the left side for a right-handed batsman), particularly leg slips (behind and slightly to the leg side, basically forming an arc away from the wicket keeper), to greatly increase the chance of getting them out caught. It also posed a safety risk, especially at the time because safety equipment like helmets were not worn at the time.
There’s a 1984 miniseries starring Hugo Weaving as the English captain, if you’re interested. I haven’t seen it myself, and tbh I’m not sure where if anywhere it’s available online, but its reviews seem very good.
The rules were eventually changed to limit the number of bouncers that could be bowled in one over, and to limit the number of fielders behind the batsman on the leg side.