Because this can still sound abstract, we break a moral act down into three specific components to determine its goodness (traditionally known as the Three Fonts of Morality):
Example: Feeding a starving person is objectively good because it directly sustains human life and fulfills the biological and moral design of caring for another human being.
Is this circular reasoning?: No, this isn't circular. Christians don't say feeding someone is good because it's "not evil." We say it is good because it is a positive realization of a real, objective design requirement for human flourishing.
The motivation behind the act must also align with goodness. If you perform an objectively excellent act for a corrupt or disordered reason, the moral act as a whole becomes corrupted.
Example: If you give a massive financial donation to a local hospital, the act is good. But if your sole intention is to blackmail the board or feed your own vanity, the act is twisted by a privation of genuine charity.
The context surrounding the act must be fitting. A good act done in an entirely inappropriate context can become disordered.
Example: Telling the absolute, unvarnished truth is a positive good. However, telling the truth to a Nazi soldier about where a family is hiding in an attic is a catastrophic moral failure, because it weaponizes a good thing (truth) to assist in a massive privation (murder).
What makes an act good is wholeness and alignment with ultimate reality. An act is good when the action itself, the underlying motive, and the context all harmonize with the nature of God (who is Goodness) and the design of creation.
Evil doesn't enter the room as a separate ingredient; an act becomes "bad" the exact moment any of those three pillars suffers a fracture, lack, or corruption.