Disney has unveiled a replica lightsaber that automatically extends and lights up from its core. To answer the question likely to be asked first: You can't buy it right now. It isn't clear whether or not the prop will ever be for sale. But you can see what it looks like in the video above, which is more or less like a working lightsaber, or as close to one as I've ever seen.
Maybe the video has been worked on to make the prop look extra good, but even so, it automatically extends and lights up brightly: two of the main things lightsabers do, along with cutting off hands and deflecting blaster fire. As Polygon (which spotted the teaser yesterday) points out, Star Wars fans have so far had to settle for retractable lightsabers that are lit from the bottom, or fully-lit sabers that look more like they do in the movies but don't retract. This new model appears to have it all.
About the lightsaber, Disney says, "Yes, that's a new type of lightsaber Rey is holding in her hand, created by Walt Disney Imagineering Research and Development," but that's about it. The prop will be used by actors at the Star Wars: Galactic Starcruiser experience at Walt Disney World in Florida, a two-night simulated spaceship stay opening in 2022. ("This isn’t storytelling—it’s storyliving.")
How the lightsaber works isn't known outside of Disney, but a 2018 patent provides the most likely explanation. The Disney patent describes a model lightsaber that operates by the same principle that allows a measuring tape to become rigid after unrolling.
The Verge explained the patent's engineering last month, and the gif below from Disney and Star Wars fan Ben Ridout illustrates the idea well. (The model won't literally use two measuring tapes, of course. The point is just that plastic strips can both be rolled up and become rigid by forming half-circles.)
Did #Disney invent a real working #lightsaber? Yes they did. It won’t melt through metal blast doors, or cut off your hand, but it does feature an illuminated blade that will extend and retract at the push of a button. This animation shows the concept behind the tech. pic.twitter.com/e7fwP06CxFApril 12, 2021
If the prop really looks as good as Disney's video indicates, people are going to want it, so I suspect there'll be a way to buy one—although maybe Disney will keep its patented lightsaber tech to itself for a few years to hype up the Florida experience.
The other question that remains is how sturdy the things will be. If all it does is nail the lightsaber look, that'd be really cool for fans and cosplayers, but it'd be far better if you could actually swing it around without the blade flopping over. Perhaps it isn't promising that the actor in the teaser video stands completely still while holding the lightsaber, but it may not mean anything.