Examples:
Roll a d20 d20
Roll a d20, but reroll on 1 (halfling d20) d20 reroll 1 hd20
Choose the greater of two d20 rolls (advantage) d20 > d20 d20!
Choose the lesser of two d20 rolls (disadvantage) d20 < d20
Roll 4d6 and keep the highest 3 rolls (common character ability roll) 4kh3d6
DC 15 check with 6 proficiency d20 + 6 DC 15
DC 15 check with 6 proficiency with 8d6 on hit (fireball) d20 + 6 DC 15 * 8d6
DC 15 check with 6 proficiency with 8d6 on hit (fireball) and half damage on save d20 + 6 DC 15 * 8d6 save half
Attack Roll against Armor 15 d20 + 6 AC 15
Attack Roll against Armor 15 with 2d6 + 4 weapon (d20 + 6 AC 15) * (2d6 + 4)
Attack Roll (with advantage) against Armor 15 with 2d6 + 4 weapon (d20 > d20 + 6 AC 15) * (2d6 + 4)
Attack Roll (with advantage) against Armor 15 with 2d6 + 4 weapon, 4d6 + 4 on critical hit (double dice) (d20 > d20 + 6 AC 15) * (2d6 + 4) crit (4d6 + 4)
Attack Roll (with advantage) against Armor 15 with 2d6 + 4 weapon, 4d6 + 4 on extended critical hit (top 2, 19 or 20, double dice) (d20 > d20 + 6 AC 15) * (2d6 + 4) xcrit2 (4d6 + 4)