A white person, or more broadly a foreigner.
It is usually neutral or affectionate rather than hostile. Children will shout it across a street, cheerfully, and traders will use it to get your attention. Taking offence is generally a misreading of the situation.
The counterpart is obibini — a Black person.
Diaspora visitors are sometimes surprised to be called obroni in Ghana. It is used for anyone read as foreign, which can include Black people whose dress, accent or bearing marks them as coming from elsewhere. It is not a statement about belonging.