What is "The Tin Drum" by Gunter Grass about?

The book is really confusing and I need to know for English. Can you please explain the summary/theme/plot of the story? And what does the "Tin Drum" represent in Oskar's life? And what is the theme? Thanks
Update: PLEASE, IF YOU READ BEFORE, INCLUDE THE THEME AND WHAT THE TIN DRUM PORTRAYS
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