Why the car is stop the red light is see ?

你是否指車尾的紅燈?
如是的話,是因為當駕駛者踏下制動器時車尾的紅燈便會亮起.
令其他駕駛者知道這車正在減速中而避免撞車.
至於如果汽車在停止時尾的紅燈仍亮起,只是駕駛者仍踏下制動器不放而已.

i think you are asking
why the red lights at the back of the car will on when a car stops, right?

Itz becoz the driver has stepped on the brake, which will stop the car and switch the red light on at the same time, therefore, drivers who are following the light on car will notice that the car is going to be stop..... becoz after stepping on the brake, it still takes time to slow down and stop the car......

is this answer fine for u?

妳不如試下用中文問啦... 我真係睇唔明妳問乜...?!