A "prohibition sign" means a safety sign prohibiting behaviour likely to cause a risk to health or safety. Display of appropriate signs prevents unsafe situations. These health & safety signs are generally need to be red and it shows only what or who is forbidden.
This prohibition sign is used to inform people to not the maintenance activities when the machine is running. There is actually a very serious threat because the moving parts of the machines as it might cause some damage to the maintenance worker. So, while carrying out maintenance activities workers should make sure that the machine is first shut down. This sign is a must for better safety compliance.