Taking your question at face value, and since the joke relies on a couple of things, and I'm not sure which is not connecting for you, I'll go into too much detail. The idiom "her face lit up" means she was delighted, such as when opening a present. When you open a refrigerator door, the light inside goes on. Thus, when she opened this present, the light shone on her face. Did one of these close the loop for ya?
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