Karma is a number. PCs don’t get to know that number.
As the rulebook states, Karma determines how people in the Fallout world perceive the PC party (and the individual PCs). The way it works in this campaign is that the party’s Karma is the average of each PC’s and NPC’s Karma. Also, there is global Karma and town specific Karma. This is also a simple average (K1 + K2)/2, etc.
If you have a global Karma of -1000, it’s because you dropped a nuke on a preschool. A town Karma of -5…you shot someone’s pet Brahman to death.
Karma determines who will talk to you, the prices of things, quests, etc. It is your ethos. -1000 Karma doesn’t mean you won’t get any quests, just that they will probably either be redemptive or damming.