eval use ?

hello,
i am just tryin to learn lisp from robert wilensky's common lispcraft.
But i am all stuck at the eval function use ..can somebody help me out
what does exactly eval is used for specially can you give me some
examples of complex code where it is being use.. coz its like i can get
the simpler examples but not gettin it utility for complex thinkgs
where it can be used..
Thanks

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