The phrase implies that someone is holding onto an idea or concern that others might find trivial or insignificant, but they are fixated on it.
Here are some examples of how you might use the phrase:
* "My neighbor has a bee in his bonnet about my dog barking."
* "She's had a bee in her bonnet about getting that promotion for months now."
* "He's got a bee in his bonnet about the government, but I don't understand his concerns."
The origin of the phrase is unclear, but it's likely related to the image of a bee buzzing around someone's head, creating a nuisance.