Without the minimum equipment lists (MEL), if a reading light on seat 36E burned out, the aircraft couldn't take off until it was replaced. The MEL allows them to fix the light within 10 days and go for now. Literally nothing on an airplane can be broken or not working without some sort of authorization, the MEL being the most common. The MEL is based on the safety design of the aircraft and takes redundancy into account. It also provides any additional inspections or operational restrictions required to operate with that item not working.
MELs can be abused, but the FAA and pilots come down hard on this. My old operator lost their ability to MEL the aux power units on some fleets because they were being used too often.
Sorry, I'll try to spell them out in the future.