Heyo :)
The "D" and "d" stand for dice. So "D6 Alley Encounters" means you roll a die with 6 sides and compare the result with the table. For example: You roll a 2. Therefore the second event is happening.
The "1" before the "d6" means how many dice you roll. 2d6 means you roll two six-sided dice. 1d6 means you roll just one six-sided die. 3d4 means you roll three four-sided dice.
I know my response is too late, but maybe someone from the future needs this explanation :)