X+Y is the equivalent of what you have set for your keys on the keyboard. Many people have
xyz = asd and abc = zxc. So X+Y would you be pressing A+S at the same time. Look at your key config in options if you're not sure.
D, D, D, X+Y is just about the same. Tap Down 3 times quickly then press X+Y. In command terms
F = Forward
B = Back
D = Down
U = Up
Sometimes they will combine into things like DF or DB in which case they are the 2 keys pressed at once. If you read readmes people will often talk about QCF which is a quarter circle forward, Equivalent to D, DF, F. Some really really weird people use numeric notation which corresponds to the numpad. QCF in that sense is 236. You are ALWAYS assumed to be facing right with any command. Some moves may not need a specific key. Readmes may refer to P or K, which means punch or kick. Or PP or KK which means any 2 punches or kicks.