Spoilers ahead for Star Wars: The Last Jedi. The Star Wars movie saga features a lot of characters who are strong in the Force, the energy that brings balance to the universe. There's Luke Skywalker and his twin sister Leia. Their father, Anakin Skywalker, plus the other Jedi, like Yoda and Obi-Wan Kenobi. Then there are the Sith, who use the dark side of the Force. And though we've seen these characters use their powers in various ways throughout the many movies, we've never seen any of them use the Force how Rey and Kylo Ren do to talk in The Last Jedi.
A good chunk of the plot of The Last Jedi hinges on Rey and Kylo Ren's ability to communicate with each other across the galaxy by using the Force. But how does it work, exactly? Essentially, they can see each other in front of them, as if they were just sitting across from one another in the same room. They can hear each other's voices, see their bodies, and vaguely make out their locations, though that part gets a little fuzzy. It's also a little unclear how it's initiated. Do they both have to want to talk at any given time? Or can one person kind of "make a call" to the other? It's not totally clear, but it is really neat, and allows for some major character and plot developments to take place.
The fact that Kylo Ren and Rey can speak to each other only naturally leads them both to a bit more understanding about each other as well. Kylo Ren tells Rey what happened at Luke's Jedi Temple, why he destroyed it, and how he feels about Luke. And Rey tells Kylo about her longing for her parents, the dark cavern she visits on Luke's secluded island, and her relationship with his father. It's all very much like a Force therapy session every time they speak, and they don't even have to actually tell each other things, visions and feelings pass across to each other as well.
This allows for the two characters to grow closer, for better or for worse. Rey takes pity on Kylo Ren, even when he explains why he killed his own father. And Kylo Ren helps Rey understand more about herself and her abilities in the Force. Their communication has its downfalls too, however. As it's revealed later on in the movie, their connection was initiated by Supreme Leader Snoke in order for Kylo Ren to figure out where Luke Skywalker was hiding. Snoke essentially used them both in order to try and lure Luke from his seclusions. After killing Snoke, Kylo Ren then uses his knowledge of Rey's inner most desires, fears, and feelings to try and lure her to the dark side. But thankfully she refuses.
But even after Snoke's death, Rey and Kylo Ren continue to see each other in their mind's eye. They don't speak again after the climax, where Kylo Ren angrily takes over as Supreme Leader himself. But the fact that they can still see each other suggests that perhaps it wasn't just Snoke's power connecting them alone. They can do it on their own after he's gone, and are both strong enough to use the Force to communicate across space. That's not a surprising talent for Kylo Ren, but it is for Rey, who basically just started using the Force five minutes ago.
But the fact that Rey and Kylo Ren could communicate throughout The Last Jedi made for some really interesting conversations, in addition to lovely visual cues, that really enhanced the character and plot developments of the movie. So it's going to be interesting to see if they continue their communication throughout Episode IX, and what that might mean for the two going forward.