From my perspective, the literal ending was that Jack went down into the glowy cave and put the rock back in the hole, then got spit out the other end somehow (same as the man in black after Jacob pushed him in after killing him). After Jack came out, he staggered to the bamboo field and in the moment of his death there, with Walt's dog next to him, he experienced the entire flash-sideways (essentially experiencing the entire alternate reality of season 6 in a single moment) and all the characters who had died in the past or would later die in the future, and who were the most important people in Jack's and each others' lives, gathered together one last time before "moving on."
Basically, it was saying that the flash-sideways took place at some unknown point in time when all the characters were already dead. Most of the mythology and questions about the island were sorta left unanswered.
I think it was a nice way for everyone on the show to gather, wrap up their relationships with each other (Sayid and Shannon, for example), and say goodbye, both as characters and to the viewing audience.
Although you didn't ask it, I'll say I didn't adore the ending, but I didn't hate it either. It left a LOT unexplained; however, perhaps if they HAD tried to explain too much, it could never have lived up to people's imaginations, so they left all the mythology and island-strangeness explanations up to the viewers. As far as "planned out" tv shows go, I've seen better endings (Babylon 5) and worse endings (Battlestar Galactica), so I don't mind Lost sorta breaking even.