Sorry, getting really off-topic here. I was just using that as a for instance but thanks for pointing it out. But... um... you mean what exactly by "negative on HIT"?
It means that on hit, your opponent will be able to act before you can.
It all boils down to hit advantage, block advantage, start up frames and recovery. Its not too hard once you grasp it, so lets get down to it...
Frames are essentially, 1/60th of a second. 60 frames in a second. Moves have start up frames, which is how many frames it takes for them to become active and ... do stuff. Some moves are fast, some are slow. Superman, that dickback, has his F23 which has 8 frames startup, that's really fast. Most D1s are 6-7 frames thankfully, but you get a few odd ones like Bane or Grundy having really slow D1s. Anyway...
When you do a move, it has recovery frames after the active frames. Where you are putting your guns away, or going back to a normal stance after punching. This varies from move to move, some recover quickly, some not. And the thing about recovery is, it tells you whether a move is safe, or not, depending on how the move is executed.
Now we move onto block advantage, and hit advantage. When you hit someone with something, it puts them in a state where they can't do anything or react for a while. On block, this is usually much lower than on hit. So, for example, if a move is -6 on block, it means that if your opponent blocks your attack, your recovery frames, will stop 6 frames after his blockstun. Giving your opponent an advantage of 6 frames. With sinestros case, his fireballs are both negative on block, and on hit.
However, you must also think about it if your move doesn't hit at all. It doesn't have any hit advantage, or any block advantage. You only have recovery frames and that is very dangerous for you.
All this is found in the move list. You will want to look at a moves start up frames, to see how quick it is. Sometimes you need fast moves to beat your opponents fast moves, how safe it is, you don't want them to punish you.
I've probably missed something out, but this probably tells you enough about frames and stuff.