The "d" is a die, as in one of several dice.

1d3 means one die, three sided. 3d6 would be three dice, six sided. Therefore the club mash does 1-3 plus 15 damage, or in other words, 16-18 damage.

More dice limits the range and centralizes the score. Let us compare 4d6 to 2d12:

4d6 has a range of 4-24, and 2d12 a range of 2-24. While the 4d6 has a higher minimum, it is also less likely to roll high, since the odds of all four six-sided dice being sixes is lower than the odds of the two twelve-sided dice being twelves.

As I said, it is a relic, and I say this because in "the olden days" there were practical consequences once weapons became stronger, and had more dice, while nowadays the computer calculates it all. Consider a weapon doing 5d20 of damage: who wants to fuss about whether all five dice were rolled fairly, then proceed to constantly be tallying them? Much easier to simply roll 1d100.

In the M&M series, these quandries are largely irrelevant since in games 1 and 2 battle is too rapid for anyone to care; in games 3, 4, 5 and Swords, the question is rendered moot by Diamond and Obsidian weapons (whose bonuses are extreme); and in 6, 7 and 8 the character's MGT score and magical bonuses create a situation where the dice involved almost never are responsible for the win or loss of a battle.