Hiccup Is Reunited With Toothless
The Hidden World is essentially only accessible if you're a dragon or flying on the back of one, since the only way to get there is via an enormous waterfall in the middle of the ocean. So, while it's not enormously far away from New Berk, Hiccup isn't able to go and visit Toothless whenever he feels like it - and perhaps preferred to make a clean break when he said goodbye. Hiccup and Astrid do see Toothless again, however, at the very end of the movie, when they make a journey with their two children to the waterfall that hides the entrance to the Hidden World.
Based on Toothless' reaction to seeing Hiccup again, it seems that this is the first time Hiccup has been to visit his old friend since saying goodbye all those years ago. Toothless doesn't recognize his old friend at first (perhaps confused by the beard), and seems to have grown more feral and wary of humans over the years. However, when Hiccup holds out his hand in a familiar gesture, Toothless suddenly recognizes him and is overjoyed to see him again, bowling him over in an excited greeting.
Toothless and the Light Fury have also been building a family of their own over the years, and have several baby dragons that are a hybrid of the two species with traits from both of them. Once Toothless recognizes Hiccup, he and the babies are introduced to Hiccup and Astrid's own children. Though this might not be a journey that Hiccup and his family end up undertaking often, it gave him the opportunity to bring to life the stories he told his children about the dragons he knew when he was a boy - so they can pass down stories to their own children as well.
What How To Train Your Dragon's Ending Really Means
The How to Train Your Dragon trilogy is, at its core, a coming of age story. It begins with Hiccup as a teenager who feels like he doesn't quite fit in, has a troubled relationship with his father, and isn't in any way ready for the responsibilities that will eventually be thrust upon him. How to Train Your Dragon was primarily a movie about Hiccup realizing that he doesn't need to grow up to be the man that his father and the villagers expect him to be, and that he can actually forge his own path - making his mark by creating peace between humans and dragons.
How to Train Your Dragon 2 and How To Train Your Dragon: The Hidden World were both about Hiccup's continued pursuit of this dream. In the second movie he was reunited with his mother, Valka (Cate Banchett), and realized that he takes after her in his love of dragons. However, he also encountered the monstrosity of mankind in their treatment of dragons, with fierce conqueror Drago Bludvist (Djimon Hounsou) weaponizing the creatures for his own means. In many ways, Drago was a precursor to Grimmel and the other dragon hunters who arrive on the scene in The Hidden World.
While Toothless was an essential part of Hiccup's journey to becoming a leader, letting go of the dragon was an equally essential step. It not only requires Hiccup to put others' interests above his own desires and ideals - recognizing that the dragons are better off on their own, however much fun it might be to ride around on their backs - but also forces him to stand on his own two feet (well, his own foot and prosthetic leg) instead of using Toothless as a crutch. A moment of letting go is a common turning point in coming of age stories, and is symbolic of a farewell to childhood and an embracing of maturity.
Beyond bringing Hiccup's character development to a satisfying conclusion, How to Train Your Dragon: The Hidden World also has a more explicit final message. As Hiccup explains, the dragons had to leave because humans' warring nature meant that they could no longer share the world with them, and they will only return when humans have learned to better themselves and stopped resorting to wickedness and violence. The How to Train Your Dragon movies are primarily aimed at a younger audience, and as a parting farewell they impart a parting moral message of cooperation and self-improvement to that same audience. And to be honest, it's a message that adult audiences could benefit from listening to as well!