What's the meaning of a jock strap?

A jockstrap is an undergarmet worn by boys and men to provide suppport during sports. A jockstrap helps prevent injury. Some jockstraps have a pocket for holding a cup. A cup provides protection for impacts (for example due to a baseball or a hockey puck). If impact protection is not needed, the male athlete does not need the cup

I think to support the cup when guys are doing sports or other activities.

Jock was US slang from about 1950, originally a shorter form of jockey but it came to mean any athletic man. Jock meant ‘John’ as in ordinary guy, Joe or bloke from the 1500-1800’s in the US (in the UK it meant Scotsman), and horse-riding US guys later became commonly known as jockeys. Bike-jockeys were the source of demand for a garment that kept “the twins” tidy when bicycle riders stood up and sat down while cycling the cobbled streets of Boston, c.1874, and this garment became the commercial jock-strap. The cup was a later addition after the garment had become popular in other sports. The design of the garment probably goes back further than the first commercial version, as you can see its ‘loincloth’ forerunners in the pioneering stop-motion photography of Muybridge c.1870. Although he was English, he did a lot of work in the US and had quite a Wild West life - including being thrown from a stagecoach; he was also acquitted by a jury for killing his wife’s lover (an army major).