before long; soon; someday; sooner or later
Expresses that something will happen in the near future, though the exact timing is unspecified. Often used when the speaker expects a natural progression or a resolution without precise scheduling.
See also: そのうち
The rain will probably stop before long.
Let's meet again one of these days.
その内 is the same expression without に and is used identically to mean 'before long', with そのうちに being a slightly more common spoken variant.
From その (that) + うち (inside/within) + に (particle), literally 'within that (time)'. The phrase grammaticalized into an adverb meaning 'before long'.