In the real world, a "maelstrom" is a powerful whirlpool.
In the universe of Warhammer 40,000, the Maelstrom (with a capital M) is a massive rip in space-time that leads to the Warp -- second only to the Eye of Terror in size -- in the Badab Sector, near the galactic core.
The forces of Chaos started using the Maelstrom as a launching point for invasions, and since there was a lot of mineral wealth in the area, the Imperium decided to post guards around it. These were the Maelstrom Wardens, four Space Marine Chapters (the Lamenters, the Mantis Warriors, and the Charnel Guards [who were later withdrawn from the Sector], all under the command of the Astral Claws).
Lugft Huron, Chapter Master of the Astral Claws, repeatedly proposed that a Crusade be mounted to purge the Maelstrom of heretics once and for all, but was rebuffed; finally, he took matters into his own hands, declaring himself the Tyrant of Badab and seizing all tithes and production for his Chapter and allies. The Imperium objected, and started the Badab War to put him in his place. Eventually, they took control of the Sector from him, but not before he and the remnants of his chapter fled into the Maelstrom, becoming the Red Corsairs Chaos Space Marine warband. After the fall of Cadia the Maelstrom is now connected to the Great Rift.
There was also a Selection of short Warhammer 40,000 stories, titled "Into the Maelstrom" with a derp ass picture of good ol' Huron on the front.
Outside of Warhammer 40k
In RPGs, Maelstrom was a decent yet relatively unknown game set in 16th century England.