'Joint' is a slang word for a place/ location such as a bar, nightclub, cafe etc. So a 'breakfast joint' could mean a place you can get breakfast. But 'joint' is also a slang word for a hand rolled marijuana cigarette, so a 'breakfast joint' could also refer to a marijuna cigarette for breakfast! I think the first definition is probably more likely to be correct but it would be easier to say if you had written the whole sentence.