When. Said to the couple at a marriage — the words the Prophet ﷺ used when congratulating someone who had married. It replaces the pre-Islamic congratulation, which wished the couple ease and sons; the sources record that wording being set aside in favour of this.
The words. The sentence uses baraka twice with two different prepositions, and the pairing is deliberate. Bāraka laka — blessing for you, in what you have been given. Bāraka ʿalayka — blessing upon you, descending on the person rather than the gift. Arabic distinguishes a blessing in a thing from a blessing on its owner, and the supplication asks for both rather than choosing.
Wa jamaʿa baynakumā fī khayr — "and joined you two in good". Jamaʿa is the plain verb for bringing together, and the qualifier does the work: not merely joined, but joined in good. A marriage can be a joining without being that.
What it does not ask for. No wealth, no sons, no specified future. The pre-Islamic formula named what it wanted; this one asks for blessing and leaves the content of it unstated.
Practical. It is said to the one who has married — the grammar is singular — and both spouses are covered by the final clause. Nothing is attached to a count or a time.