Mission critical

Mission critical means something is really important and has to work correctly for everything else to work. Imagine that you had a really big LEGO house and it needed a lot of bricks to be built - those bricks would be mission critical because if you don't have enough of them, the LEGO house won't be built properly. Something else that could be mission critical is your computer. If it doesn't work properly, you won't be able to do anything else.