“It's a little overcast.”
Your husband is getting dressed to leave for work. You're sitting next to the window, so he asks you if it looks like it will rain today. You tell him that it's cloudy.
It's a little overcast.
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When the weather is "overcast", it means that there are clouds covering the sky. It's not raining (at least, not a lot) but you can't see the sun.
It is common to talk about the weather with phrases starting with "It's":
It's nice out.
One meaning of the phrase "a little" is "slightly" or "somewhat":
I can tell that you're a little scared.
I'm a little sleepy.