Soul to me is when a piece of art is greater than the sum of its parts. It is the most important fundamental in any art form because it is able to evoke in the viewer feelings of happiness, sadness, lust, comfiness, and even more abstract or material feelings such as making the viewer feel like they're really standing on a painted dock as the ocean mist falls gently on their cheeks; for example.
Simply, it just means you like something if you think something has soul.
I'd like to think that most artists have the ability to put soul into their work simply because most humans have been blessed with that very nature. It's not something beginner artists should worry about or find ways to boil it down to a mathematical equation. When it comes to soul, I think the biggest barrier is lacking the skill required to put their thoughts onto the canvas or having so much technical knowledge but not enough courage in their passion for art that they focus entirely on making a cold, rigid
piece because they've beaten into themselves that the practical fundamentals are something you learn first and style way, way later.