Mission critical

You need a good explanation for the word “mission critical” for you manager and an example, why redundancy is a good thing? Okay … i have one … ISS toilet fails to suck. The toilet on the International Space Station is out of order and - to add insult to injury - it´s the only one. Toilets are absolutly mission critical … well, you can´t open the airlock to take a walk behind the next tree ;)