The simplified answer is: "POSITIVITY".
But there is no single exact definition. It's about anything good, celebration of life, finding inspiration, enlightenment and personal growth, kindness and compassion, tolerance and acceptance, and making the world a better place. This open concept allows OMANI to mean many things.
Some common themes are Peace, Love, Happiness, Unity, Consideration, Empathy, Honesty, Patience, Forgiveness, Humility, Goodness, and Compassion.
OMANI comes from the first three seed syllable sounds of the compassion mantra:
Om Mani Padme Hum