Shou Sugi Ban (焼杉板, pronounced Show-sue-ghee-bon), also known as Yakisugi-ita or more commonly just Yakisugi in Japanese, is a term used to describe the ancient Japanese art of preserving wood by charring it with fire. As yaki (焼 read as shou or xiao in Mandarin Pinyin) generally means burnt or heat-treated, sugi (杉) is cypress - also known as Japanese red cedar, and ita (板 or ban, again in Mandarin) means board or plank, the phrase's literal translation is quite elementary. Shou Sugi Ban is the term more commonly used in China and Western countries for this process, and not one that would be understood by most native Japanese. But out of deference to the people and culture who developed it, I will hereafter refer to it as Yakisugi. For a comprehensive explanation of how and why the misreading of Yakisugi-ita as Shou Sugi Ban is even a thing, please refer to the Nakamoto Forestry link in the Sources section.
The heat treating of wood as a "technology" has been known to humanity since prehistoric times, and has most commonly been used at an industrial scale to waterproof wine and whiskey barrels, fence posts, boat hulls, and exterior siding. So while the the concept of charring wood to seal it is not distinctly Japanese, the process that the Japanese developed is what the modern market for construction materials is based around. Further, industrial Japanese forestry has created an international product wherein they mill, kiln dry, and treat the timber prior to sale, so you can buy your lumber already charred to a uniform level and not have to deal with potentially setting fire to the construction site. The biggest Japanese manufacturers of Yakisugi are all located in Western Japan, with the biggest producer being Nakamoto Zourin in the Hiroshima prefecture. In Kyoto, where construction with these materials is still very common, the process is also known as Yakiita. In most cases, automated machinery is used to burn the wood, but a few manufacturers still use the manual, traditional process.
Following the traditional method, three boards of suitable width are bound together tightly using wire to form a triangular prism shape, then stood upright with the open ends at the top and bottom. Newspaper or a similarly inflammable material is inserted into the bottom end and ignited. The fire will draw up quickly due to the chimney/stack effect, taking only a minute or two for the inside of the prism to become a column of flames. Finish time depends upon the moisture content of the boards combined with the desired char depth, but five minutes is typical for a 3-4mm surface burn. The heat inside the prism must exceed 250°C but usually reaches >400°C. The prism is opened and the fire is extinguished with water once the desired grade of charring has been reached. A video demonstration of this technique featuring Japanese architect Terunobu Fujimori may be found here.
Thanks to its "all natural" qualities, sustainability, environmental friendliness, durability, and aesthetic appeal (a dark patina often with silver or iridescent streaks), the burnt wood finish has become something of a design trend over the past few years. The fact that it's also easily done by any DIYer with a propane torch (and hopefully a fire extinguisher) adds to its appeal. Applications of Yakisugi are not limited to building materials for exterior use like decking and siding, but are frequently found in the treatment of small pre-assembled wood furnishings, including shelves, tables, and chairs. The method is not limited to cedar, and can be used with a variety of softwoods including pine and fir. Hardwoods are generally avoided due to their density and resistance to charring, but not entirely as there's a wide range of darknesses that are possible for the final result. Hardwoods can take a light brown singe that leaves much of the wood grain untouched, where softwoods can reach the jet-black of charcoal. Lighter treatments are typically followed with one or more coats of linseed oil, tung oil or other natural oils suitable for sealing wood. For a general guide on how to do it yourself, refer to the Bob Vila link in the Sources section.
Sources:
https://shousugiban.com/
https://japanwoodcraftassociation.com/traditions/techniques/yakisugi-shou-sugi-ban/
https://nakamotoforestry.com/yakisugi-or-shou-sugi-ban-learn-what-you-should-call-it-and-why/
https://www.bobvila.com/articles/burnt-wood-finish/