A.K.A. everything about Saltmarsh's history that would be super-common knowledge to anyone in the area
(You're not expected to know or memorize any/all of this, but it's here if you're interested)
Saltmarsh is a small fishing village on the southern coast of the Kingdom of Keoland. Until recently, the only thing of note besides the fishing industry, was that it was a good spot to stop if you were travelling West from the provincial capital. The Kingdom itself paid little attention to its southern regions, focussing instead on a series of expansion wars in the North, East, and West.
In the last few decades, those expansion wars have mostly ended, with Keoland losing almost all of the territory it had briefly gained. Veluna, Bissel, Gran March, Sterich, The Lost Lands, The Yeomanry, and the three Uleks all once were owned, at least in some part, by Keoland, and the kingdom is now perhaps a quarter the size that it was at its height. While the crown's focus was elsewhere, the southern coast fell prey to smuggling, piracy, banditry, other crime, unchecked monsters, evil cults, and all the other sorts of things you get in the wilder parts of a D&D world. An entire nation of pirates, the "Hold of the Sea Princes", has formed, and been flourishing for over a century, raiding Keoland's coastline with little response
Six years ago, King Trevlyan III passed away without any heir. The Council of Niole Dra chose Kimbertos Skotti as the new king. In an effort to recover from the recent losses, King Skotti has decided that Keoland will be a mercantile nation, instead of a military power. Per the GoS book, "If Keoland cannot prosper as a military force, it must grow mightier as a center of trade."
King Skotti has now signed peace treaties with most of Keoland's former enemies, and the Royal Keoland Navy have been raised in both size and funding, allowing him to begin countering the Sea Princes' ambitions. But Keoland won't be able to start dealing in large-scale trade with other nations unless they can expand their ports, and work to secure the seas even further.
While the provincial capital of Seaton to the East is already much bigger than Saltmarsh, several things have led to Saltmarsh getting much of the Crown's focus in the area. First, the nearby hills and seaside cliffs have shown signs of ore deposits, and a band of dwarves, bearing a decree from King Skotti himself, showed up and started up a mine. Second, The Styes, the port district of Seaton, is a run-down, poverty-stricken, toxic mess. The alchemists' quarter have been using the harbour as a place to dump out failures and spare reagents for far too long. Lastly, Saltmarsh is a mere fishing village, and has little pre-existing political power. This means the Crown as been able to influence things more directly than they might get away with elsewhere.
The result is a town that has become politically divided. Some believe that the Crown's attention can put a stop to smuggling, bring in more population and revenue, and breathe new life into Saltmarsh. Others see the Crown as meddling, fear that the Crown's influence will destroy their way of life, and just want to go back to living in a quiet fishing village. Many in the town aren't originally from Saltmarsh; the dwarven miners are new to the area, as are many of the former soldiers who have recently been added to the town guard. And, of course, the town's more criminal elements are simultaneously worried that the Crown's attention will bring down the law, but also keenly aware that it might bring new opportunities, as well.
And so, with money flowing through, political divisions causing strife, and plenty of smugglers and pirates in the picture, Saltmarsh has attracted the attention of four mercenary adventurers...