What does it mean when it says danger 415 volts?

It means there is a serious hazard to life safety due to the presence of high voltage, which may require additional worker training and protection for even those authorized to enter and start touching things, let alone ignorant intruders, who should be excluded by suitable locks and barriers.  Ordinary hazards with high voltage (and accompanying high power) include not just electrocution or shock, but also arc-flash  burns, thermal burns, and possible blast injuries.

It means stay away.  You can be KILLED.

Danger means Danger.  I don't care why.

The electric supply is 415 volts (3 phase) and therefore dangerous.  

Time to work on reading comprehension. 

Stay the **** out for safety reasons.

duh .. there's electricity at 415 volts inside.  for reference, 415 volts is sufficient to kill you at the usual amperage used.

There is probably a blast proof fan installation.  These usually run on 3 phase and are deemed uninterruptable as a supply. The higher voltage is much more dangerous to be around than regular domestic supplies and can be lethal.

It's a standard warning. Equipment in the building uses 415 volts, which is dangerous if you don't know what you're doing.

It means come in and swing an iron chain randomly into everything you see

Danger, 415 volts is enough to kill you. Single phase used by the electric utility for lighting and killed a boy near me when he touched a light pole on a bridge. There wasn't any Danger sign on the light pole.