out in the open: visible, evident, and publicly known, as opposed to hidden or secret
Phaneros belongs to a family of Greek words built on the idea of shining or coming into view (the same root gives the verb "to bring to light"). At its core it describes whatever is out in the open rather than concealed. Depending on what is in the open, English reaches for different words: a fact that is plainly evident, a person or piece of news that has become widely known, or a deed done publicly rather than in secret. The three senses below are simply that one idea applied to a thing, to a person, and to the manner of an action.
The word almost always defines itself by its opposite. Again and again it stands directly against "hidden" or "in secret": nothing covered up that will not be brought into the open, secrets of the heart laid bare, giving done in secret and rewarded in the open. To call something phaneros is to say it has crossed the line from concealed to exposed.
Because of that, the word carries real weight in the New Testament's talk of judgment and revelation. When Paul says each person's work will be shown for what it is, or that what can be known of God is plain to everyone, he is using this everyday word for a solemn point: on the last day, and in the gospel now, what was hidden is dragged into the light.