Vulkan He'stan is the Forgefather of the Salamanders chapter of Space Marines, once the Captain of the Fourth Company. He should not be confused with Vulkan, the Primarch of the Salamanders; He'stan used to have a different first name, but he gave it up for Vulkan's when he became the Forgefather.
Part of his duty as Forgefather is to seek the artifacts that Vulkan made and scattered throughout the galaxy. In particular, four remain to be found: the Engine of Woe, the Obsidian Chariot, the Unbound Flame, and the Song of Entropy. His task would be made slightly easier if anyone alive knew what these were, but apparently, Vulkan only left the names. Vulkan spent some time in a casket inscribed with the words Unbound Flame, but he did not make the casket, it is on Nocturne, not lost, and its nature is known to both the Salamanders and the Ultramarines, rather than a total mystery.
Fortunately, He'stan wears three of the recovered relics to help him on his way: the Gauntlet of the Forge (which has a built-in heavy flamer), Kesare's Mantle (which gives him a 3++ invulnerable save), and the Spear of Vulkan, a master-crafted relic blade.
He also speaks Dark Eldar fluently enough that a Haemonculus was surprised by it.
In addition to having the statline of a Brother-Captain (with a 2+/3++ instead of a 3+ armor save) and all his tricked-out wargear, Vulkan He'stan confers a great advantage on the army that he leads: all melta and flame weapons in a unit he focuses on that meets some criteria loosely fitting "any non-centurion infantry, any bike, or any dreadnought" re-roll to both hit and wound. For this reason (not to mention being one of the Salamanders, and therefore one of the few genuinely good guys in the Warhammer 40,000 universe), he is a popular choice to lead Space Marine armies.
A common strategy is to field a list full of multi-meltas and heavy flamers packed into drop pods when using him. This is because using a melta becomes a lot more reliable when using Vulkan He'Stan, meaning you can count on the meltas to at least hit (and we all know what happens when a meltagun hits). Also of note is that he makes stock Dreadnoughts incredibly efficient when used in a drop pod. If you upgrade the dreadnought to Venerable, you literally have a 2.78% chance of missing the target. Great for when you absolutely have to kill that pesky Land Raider.
Another thing of note is that despite his army-buffing prowess (the biggest reason most people take him), he's got great prowess of his own. While having "only" the stat-line of a Captain, he's actually a steal in terms of points. A Salamanders captain somehow allowed to buy all of his gear at once with costs scaled to the captain's equipment would be 85 base + about 7 for the force axe, based on his costs of 5 for a power axe, which is worse, and 10 for a relic blade, which is much better + about 10 for the flamer, since it's on par with a combiflamer + about 7 for the combat shield presumably granting him a 2+ = 109 points, and he'd still be stuck at a 4++; you'd need to upgrade him to a full-on chapter master to also include something like his buffing, which would spike you to 149.
In the Fluff
Yes, he makes an appearance. Where? Why, in the pages of Nick Kyme's Tome of Fire series, of course. He spends most of his time being a boss and impressing the Fire Drakes, frightening a Homunculus, and eventually leading a boarding action against the Red Dragons Chaos Space Marines. The main man himself sticks around to help explain how some Dark Eldar technology works, and demonstrates how a Webway Mirror functions on science and not Warpcraft when it's being used on him. However, because the Fire Drakes job hard in this series, it does help him look even more badass when he takes on a Soul Grinder in close quarters and takes it the fuck apart. At the end of the series, he decides that he needs to go find the Unbound Flame, one of the missing Artefacts of Vulkan, and sets off.