You're having lunch with your friend at a restaurant. You're looking at the menus to decide what to order. Your friend asks you what you're going to get. You say:
I'm in the mood for something light.
A "mood" is a temporary feeling that you have, which normally lasts a few hours. For example:
Don't talk to her now. She's in a bad mood.
In this example, you're saying someone has been angry or upset for a few hours or for a day or two.
When you say "I'm in the mood for ___", it really just means "I want ___". But saying you're "in the mood for" it makes it sound like it's a temporary feeling that might be a little different from what you usually want.
In the example at the top, saying that you're "in the mood for something light" means that you want to eat light food now, although you may usually get something heavier.
When you want to talk about a thing that fits a certain description, you can use "something ___". For example:
We should do something fun this weekend.
Food that is "light" usually doesn't have much fat or calories. Light foods include salads, fruit, some sandwiches, etc.
Foods that aren't light can be called "heavy" food. Examples include steaks, fried foods, creamy pasta dishes, and so on.
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