Nonduality means literally not two. What this is saying is that, in reality, though it may appear that life is full of seemingly separate independent forms, there is one fundamental reality which runs through them all. Nothing is actually separate, that is an illusion, a mirage. All arise from one source and remain always as That, even though it may appear otherwise.
Why does this matter? Because the moment you imagine yourself to be separate, you have cut yourself off from your truth, your love, your wisdom, your power. From then on, you have limited yourself from the whole and then look outside yourself and imagine you're seeing a foreign and hostile world which you must contend with for your survival. When the body that you take to be you, dies, it’s believed that you die with it. And so this idea of separation generates all sorts of beliefs based on fear and limitation.
When all that’s seen is yourself, there is no longer any need to fear. There is no longer limitation, because there is nothing that is not you. There is nothing to do, nothing to fix, nothing to become, you can finally rest at ease, you are perfect as you are, and you give life the freedeom to be as it is.