It's pretty simple. Let's say you do the action:
Reach into the crack in the asteroid and carefully grab Tal's head with my robotic pincers, pull it out without damaging it and place it in my cargo pod.
This action is associated with robotics, so let's say you have 0 robotics. You get a roll of 1, 2, 3, 4, 5, or 6 based on your robotics skill because the action is robotics-based. +0 means everything stays at what it is: 1-6. Only 5 is really desirable, 4 is partial failure/partial success, etc. A 5 is an instant success, but let's say you roll a 4. I could write:
You reach into the crack and grab Tal's head, and just barely manage to grab it, dragging his face across the rock roughly as you pull it out - you have a feeling he won't be quite as pretty anymore when you're done with him. When you go to put it into your cargo pod, you fumble with the pincers, trying to get them to behave - you add too much power, though, and Tal's head comes loose, floating away into space. Well... at least it's out of the crack.
If you got a 3, I could write,
You reach into the crack with your robotic arm and, on your first attempt, manage to accidentally crush his jaw. At least he's dead, right? Wincing slightly, you reach in again and manage to jiggle him farther out... but when his head gets wedged in there, you decide to stop and think it over for a moment.
Or a 2:
You try to reach into the crack, but the claw doesn't quite fit. Maybe if you tried a little more carefully to get the positioning just right...
Now let's say you got a 1.
You reach into the crack a little too excitedly and the pincers get jammed. Tal's head is nowhere to be seen. After some careful maneuvering of both the arm and the thrusters, you eventually manage to get your robotic arm unstuck - but realize to your horror that now Tal's head is crushed into a hardly recognizable paste. It's his own fault for being dumb enough to try to launch himself off his fore thrusters, but still... this is truly a tragedy.
And his own damn fault.
So, that makes sense, right? With me so far?
Let's say you have a -2 in robotics. If you try to grab Tal's head, the possible rolls will look like this: 1,1,1,2,3,4. That's hardly any chance for safely retrieving my head.
If you have a +2 in robotics, if you try to grab Tal's head, the possible rolls will look like this: 3,4,5,5,5,6. That's a huge
chance of safely retrieving Tal's head with no negative side effects - 50% (67% if you count the 6, too, which is an overshoot). If you get up to +5, you'll have a 100% chance of safely retrieving it.