We call something “good” if it meets a real need that we have as an individual, a society, or a species.
The key words here are “real” and “need”.
The context of “need” is life itself. Life implies need. One evidence of life is an organism’s activity to fill a need. An amoeba extends its pseudo-pod seeking food. A tree grows roots into the ground for water. A flower opens and twists to face the Sun. A newborn baby gasps for air and cries out for warmth and food.
The meaning of “real” is also key. We may want cake, but we really need food. Many things that “feel good” or “taste good” are actually bad for us. So “moral good” cannot be determine from pleasure.
Nor can it be determined by the avoidance of pain. Many things that are painful, like removing a splinter or applying antiseptic, may be necessary to our well-being. Childbirth, while painful, is essential for our very existence.
The other side of the definition is what is “morally bad”.
We call something “bad” if it unnecessarily harms the person, impairs cooperation, or endangers the species.