dd deletes the line under the cursor. "dd deletes something into the d register (the " is part of the command that selects the register). "d selects the register. The second d is the delete command. So you could do something like "dd2w to delete the next two words and save them into the d register. The same thing works with any named register, so a less confusing example might have been "cd2w to delete into the c register.