Indicates the speaker did, or is doing, something in the second clause because they are worried or afraid that what is mentioned in the first clause has happened or will happen.
Example
주말에날씨가나쁠까봐걱정이에요.
I am worried that the weather would be bad in weekend
중요한일을잊어버릴까봐수첩에항상메모해요.
I always write memo because I am afraid that I would forget important things.
살이찔까봐기름진음식안먹어요.
I don't eat fatty food since I am worried that I would gain weight.