There are several themes in The Diary of a Young Girl, also known as The Diary of Anne Frank; it is mostly a book about growing up, about self-doubt and self-discovery, about a young girl's quest for identity, but, most importantly, it is a book about the meaning of freedom and the complexity of human nature.
Through Anne's writings, readers become indirect witnesses of the horrors of WWII and the pain and suffering that the Jewish people endured. Anne represents all young children that were forced to live in a world of war, chaos and secrets; the children that were forced to grow up much too soon and the children that learned how cruel this world can be.
Anne feels lonely and misunderstood, similar to how many teenagers her age can sometimes feel, which is actually the main reason why she decides to start a diary—to write about her deepest thoughts and emotions. Unlike most teens, however, her life is in constant danger and she and her family must hide to protect themselves from the evil people who were convinced that they are more superior than others. Despite this, Anne never loses hope in humanity and believes that there's good in everyone and dreams of a better life, a life outside the Secret Annex; in this context, hopes and dreams is also one of the themes.
It's important to mention that this is essentially a memoir; it's not a classic novel that has themes and motives, it is simply a diary that reveals parts of a young girl's mind and soul during a time of conflict and suffering.