You're a teenager. Your friend got both of you tickets to a concert that you wanted to go to. You shout this when she tells you.
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This is a very casual slang expression. It means something similar to "great" or "wonderful". It's used mostly by people in their mid-30's or younger, but it doesn't have any negative associations.
"Sweet!" can be used by itself like in the example above. Or it can be used in a sentence:
That is sweet!
By the way, this use of "sweet" is different from when you call a person "sweet". For example:
You are so sweet.
When you call a person "sweet", it means that they're nice, kind, thoughtful, and so on. When you call a thing "sweet", it means you're excited about it.