A seed vault was a place where seeds were stored long-term.
Since the 23rd century, the main seed vault for the Federation was housed in the USS Tikhov. Starting in the 27th century, Federation planets took turns keeping watch on the vault. In 3189, for the first time, the Barzan had the watch. The only access to the vault was through the transporter.
In that year, the USS Discovery was sent by Starfleet Command on an away mission to the Tikhov. There was a crisis with the Kili refugees who were dying from an infection caused by consuming mutated plants on the planet Urna. At the vault, a team from the Discovery retrieved a sample of a protein from the unmutated form of the plant. From this sample, an antidote was synthesized and distributed to the Kili. (DIS: "Die Trying")