It is from a Christian hymn and has the sense of at some point in the future. It’s a poetic term for eventually and at long last. In the sweet by and by We shall meet on that beautiful shore In the sweet by and by We shall meet on that beautiful shore

Time goes "by". "By" is the preposition you use to describe a passing object. For example, "did you see the car that just drove by?" So "by and by" creates an image of something passing without specifying exactly what it is that is passing. The implication is that it is one's life, or time itself, that is passing.

by and by its eternity and beautiful shore is heaven I love this song

The future