This is the 30th most frequent Polish word.
bo
"Bo" means "because" in English.
In this sentence, 'bo' introduces a causal relationship explaining the reason for not going on the trip.
Nie poszłam na wycieczkę, bo padał deszcz.
I didn’t go on the trip because it was raining.
In this context, 'bo' is used to justify an anticipatory action based on a possibility.
Kupiłem więcej chleba, bo może ktoś do nas przyjdzie.
I bought extra bread because someone might visit us.
Here, 'bo' again indicates a reason: why the subject wanted to leave earlier.