Well, let us start with the main theme of the whole book: "survival is insufficient." Everything from Mandel's tale stems from there.
In Station Eleven, we learn about the day that our civilization and all of society stops due to a horrible pandemic called the Georgia Flu. The world ends with the spread of this horrible sickness.
No more diving into pools of chlorinated water lit green from below. No more ballgames played out under floodlights. No more porch lights with moths fluttering on summer nights. No more screens shining in the half-light as people raise their phones above the crowd to take photographs of concert stages.
Kirsten Raymonde won't ever forget that night, because Arthur Leander (who was a quite famous actor at the time) collapsed on stage that night from a heart attack.
Kirsten continues her story twenty years in the future as she goes from settlement to settlement with a traveling group of actors and players from this vastly altered world. As part of "The Traveling Symphony," Kirsten has devoted herself to helping art survive (especially in the form of drama and music). It is this group that has the motto from my beginning paragraph: "survival is insufficient."
It is in the settlement of St. Deborah where they are almost disbanded by a prophet. Meanwhile, the reader is taken (through flashbacks) back to the former world (before the flu pandemic). Why is this important? One word: relationships. Material things are unimportant. What makes us human is our relationships with one another. Each character in this book is connected with each other character in some way.
Throughout the novel, sometimes we learn the relationship (spouses, lovers, parents, friends, sister, brothers, guides, etc.), while other times they are kept from us as readers. This suits the tone of a post-apocalyptic novel such as this one.
The most prominent symbol is Station Eleven itself, which, in the novel, was originally the title of a comic book written about the world after a fatal alien attack. Both the people in the comic and the people existing after the Georgia Flu are caught between past and future, looking back and looking forward.