edge canceling is when you touch the corner of a ledge and land but never enter the standing animation; so essentially you land on the ground but you slide off the edge. This allows you to end the special fall animation caused by recovery or other special moves (like fox and falco's illusion) and enter a normal fall animation where you are free to act. This is not a perfect explanation of how it works, or even how to do it, but you can practice by doing aerials on the corners of platforms and edge cancels the aerials just to get a feel for how to edge canceling dolphin slash.
However, 9 times out of 10 you'll probably want to sweet spot the ledge with marth as landing on stage is very punishable and other options are usually sub optimal.
