Dice are written in standard dice notation: NdX+M, where N is how many dice you roll, X is the number of sides, and M is a flat modifier. 2d6+3 means "roll two six-sided dice and add 3." A negative modifier such as 1d20-1 subtracts instead.
The d20 drives the three core rolls in 5e — attack rolls, ability checks, and saving throws. When you have advantage, roll two d20s and keep the higher result; with disadvantage, keep the lower. Advantage is worth roughly +3 to +5 on average, and it helps most when your target number is around the middle of the die.
The smaller dice (d4–d12) handle weapon damage and healing, while the d100 (percentile) is for random tables. To roll ability scores, use the 4d6 drop lowest preset: it rolls four d6, discards the lowest die, and sums the rest — averaging about 12.2 per score instead of the flat 10.5 of a single die.