Soft resetting is something that allows you to return to the home screen of the game. Its basically a reset button for the game. It can be done by pressing start/select+R+L. The purpose of this is to do something that is luck based over and over again quickly and efficiently. For example, if you want to find a certain stationary Pokemon to be shiny, you can keep resetting and then eventually after a large number of tries, you may find it shiny. Its faster because instead of turning off the game and then turning it back on, you can just go back to the home screen of the game.