A satisfying end to a novel often ties up loose ends. It resolves the main conflicts that have been driving the story. For example, in a mystery novel, the end reveals who the culprit is and how the crime was committed. It also gives a sense of closure to the characters' arcs. The characters should have grown or changed in some way throughout the story, and the end shows the final state of their development.
A great end for a novel can be one that surprises the reader. It could be a plot twist that makes the reader rethink everything they've read up to that point. Think of 'The Sixth Sense' - if it were a novel, the revelation at the end completely changes the way you view the whole story. This kind of ending keeps the reader engaged right until the last page.