A soft bad ending is basically an escapable scenario that would usually be met with a 'Game Over' screen. A normal bad ending would end the game, and you would either have to start over entirely or from your last save, while a soft bad ending can be escaped, though it might be hard, depending on your stats, and can also be escaped by loading the latest save before the soft bad end, or undoing actions. Soft bad ends can have one or multiple ways to escape.
Not good at explaining, but hopefully this helps you understand <_<