There's lots of people who don't know what the phrase `mission critical` means.

It means: `(of hardware or software) vital to the functioning of an organization.`

Lots of companies are doing things that are not of vital importance; they don't control an Air Traffic Control system.

But your employer is asking whether you'll treat the their systems `as if` they are of vital importance. In other words will you treat their projects with care just because it's important to the business.

The employer thinks they'll be able to tell from your answer how much you cared about what your previous employers were doing and therefore how you'll treat their software and hardware.

There's nothing inherently wrong with thinking that all your work is mission critical. That indicates the assumption that your employer only assigns you tasks that you treat very seriously; that could be mostly true with some jobs.

For a Q&A website a mission-critical area would be the webs-site. A non-mission-critical task might be adding helpful comments to question to know the difference.

In the phrase `mission critical` the word `mission` refers to the mission of the organization (your employer) not the mission of your users.