In the reputation tab of your profile, you'll see a green number and a red number for each question you've asked and answered. On one particular question, I have two upvotes, but the numbers in question say 30 (green) and -2 (red). What do they mean?
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It means that you had 3 upvotes (+30 rep) and one downvote (-2 rep), which adds up to a cummulative 2 upvotes, as shown in the question.
But you earned +28 rep from this question.
Additional note: if you hit the rep cap, you'll see some votes with no rep gained, and other votes where you didn't gain as much rep as you would have otherwise. For example, "3/10" means "you've received 3 upvotes, but only gained 10 rep due to the cap" and a bare "8" means "you've received 8 upvotes, but none of them gained you any rep."
The "acc/15" in yellow means an accepted answer which always earns you 15 rep, regardless of cap. (Likewise "bounty/300" or whatever the bounty for an accepted bounty answer happens to be.)