For the number thing, 1D6 means the roll requires 1 six sided die. 1 denotes number of dice, d means die, 6 is the number of faces. If I said to roll 3D4, you would roll 3 four sided dice and add up the results. Advantage means roll the die twice and take the higher value, while disadvantage means you do the same thing, but take the lower value. As for when good turns bad, it just depends on what you're doing and what your character is good at/their stats. You can roll high, but still fail, or roll low, but still succeed.