About the dice
The d4, d8, d10, d12 and d20 are the shapes tabletop role-playing games made famous; the d6 is the cube from every board game. Each roll here gives every side the same chance.
With more than one die the total stops being even: with two d6, 7 comes up most (6 ways out of 36) and 2 or 12 least (1 way each). That's why the middle totals feel lucky.
How does the Dice Roller roll the dice?
Choose 1 to 6 dice and a type (d4, d6, d8, d10, d12 or d20) and press Roll. Your browser draws a random 8-character code, and the link becomes /dice/ followed by the spec, such as 2d6, a dash and the code. The tool name, the code and the spec are hashed into a seed for a mulberry32 generator. Each die takes one number between 0 and 1, multiplies it by the number of sides, rounds down and adds 1. The total is the plain sum of the dice.
So a d10 here reads 1 to 10, not the 0 to 9 printed on many physical d10s, and there are no modifiers, rerolls or keep-the-highest rules. Because the spec is part of the seed, 2d6 and 3d6 with the same code are unrelated rolls. The link is the whole record: open it anywhere and the same numbers come up, with nothing stored on a server.
Limits: these are pseudo-random numbers, fine for board games, tabletop sessions and classroom probability, not for gambling or anything with money on it. Rolling again makes a new link, so when playing remotely, agree that the first link sent is the roll that counts.