Each die is rolled with your browser's random number generator and every side is equally likely. Pick how many dice and how many sides, add an optional modifier such as +3 for a tabletop attack roll, then press the button. Standard notation like 2d6+3 means two six-sided dice plus 3.
Yes. Every side has the same chance, and each roll is independent of the last.
The letter d followed by a number is a die with that many sides. A d20 is a 20-sided die, used for attacks and checks in Dungeons and Dragons.
Seven, which comes up on 6 of the 36 possible combinations (about 17% of rolls).
See also: Random Number Generator · Dice Roll Wheel · Coin Flip Wheel