What is meant by intrinsically safe?

The theory behind intrinsic safety is to ensure that the available electrical and thermal energy in the system is always low enough that ignition of the hazardous atmosphere cannot occur. This is achieved by ensuring that only low voltages and currents enter the hazardous area, and that all electric supply and signal wires are protected by Zener safety barriers. Sometimes an alternative type of barrier known as a galvanic isolation barrier may be used.

for example........

An intrinsically-safe walkie talkie radio, which has been designed and tested to not become an ignition source in a flammable atmosphere. Test standards may specify combinations of internal failures which may be present while still passing the ignition test.