It's used to describe a situation that has some minor difficulties but is positive overall. For instance, you could use it when a test was a bit hard, but you think the result will be fine.

Although it looks similar to ⛅ (sun behind cloud), 🌤️ carries a much more positive nuance. It's a very effective emoji for showing an optimistic attitude that, despite minor worries, everything will turn out well in the end.