A phrase used after a verb's negative form meaning 'before (something happens, often undesirable)' or 'while (the negative state continues)'. E.g., 'before it starts raining.'
雨が降らないうちに買い物を済ませておこう。
Let's finish the shopping before it starts to rain.
忘れないうちに今日の予定をメモしておく。
I'll write down today's schedule before I forget.
病状が悪化しないうちに医者に診てもらったほうがいい。
It's better to see a doctor before the condition worsens.