Star Wars: The Last Jedi was a unique film in that the story ends not with our heroes, but with complete strangers. But director Rian Johnson says it was very important for the film to end the way it did. As our main characters are all escaping on board the Millennium Falcon, the camera pulls back to create a group shot of everybody on board. It looks like it will be the moment where the film cuts to the end credits, but instead, we get a brief sequence with children on Canto Bight who are reenacting Luke Skywalker's stand on Crait with homemade action figures. Rian Johnson says that these final moments are important because we needed to see how Luke's decision was making an impact on the galaxy. According to Johnson...
That was something I really stuck to, and believe me, we went back and forth in the editing room. In the script, when I wrote that scene in the Falcon, I wrote the words, 'this seems like the perfect image to end on.' To me, it was really important to have that final scene, because it turns what Luke did from an act that saves 20 people into an act that inspires the galaxy. The notion that what we're setting up here is something big in the next chapter. And when Leia says, 'we have everything we need,' she's talking about everyone on the Falcon, but also about what we see next, which is we now have a galaxy that has seen this beacon of hope and is getting inspired to fight the good fight.
Rian Johnson tells Empire that even he considered ending the film on board the Millennium Falcon, but that he felt strongly the extra scene was needed. It acts a response to Leia's statement that the Resistance has everything they need. Earlier in the film, nobody responds to the call for help, there's a feeling that hope has left the galaxy, but Luke's act of defiance has changed that, inspiring people in even the darkest corners of the universe. We see that in the final moments of Star Wars: The Last Jedi, and we also see that some of those who have been inspired will bring useful abilities to aid in the fight.
Of course, how that final scene will really translate into action in Star Wars: Episode IX is really anybody's guess at this point. The script that they originally had for the film was thrown out and a new script, co-written by director J.J. Abrams is now being written. It's unknown if this new script will modify previous ideas, or be something entirely new. It does seem that this trilogy is being handled a piece at a time, rather than as a single story, so really anything could happen.