Take tall & short.
Duality: if x is tall, x is not short, & vice versa. You can't be both tall & short at the same time.
Non-duality: tall & short are not mutually exclusive. x can be both tall & short.
Take a 5 foot tall man.
In a dualistic framing, he's either tall or short. No two ways about it.
In non-dualism, he may be both tall & short. E.g. tall standing beside a buncha kids, short standing beside the Harlem Globetrotters.
Duality forces a binary choice between opposites, while non-duality allows apparently opposite qualities to co-exist.
Good & evil is a powerful example.
In dualism, you're one or the other. It's black & white. Good folks over here, evil folks over there. With a bright line between them.
In non-dualism, you can be recognized as both. Good in some respects. Evil in other respects. Even a single act can be appreciated as both good & evil. E.g., you save a child from drowning. Good - you generously saved a life. Evil - the person you saved will go onto become a murderous tyrant. Non-dualism does not force us to identify as one or the other. Instead, it allows us to recognize these opposing qualities at play within the individual.
Regimes favor dualism to separate the loyal from the disloyal, the favored from the unfavored, the us from the them. It's how regimes maintain their mandated order.
In honest & loving relationships, we embrace our irreducible complexity, and we honor that no conceptual box ever comes close to capturing a being's essence. Non-dualism embodies this all-accepting approach far better than dualism does.
Better than both dualism & non-dualism, though, is letting conceptuality go altogether, and appreciating everything as it is, without any need for labelling or categorizing. You might call this "naked awareness" or "being free from concepts." This is the state of clearest seeing, deepest peace, and flowering joy.