Othello is a Moor, which is to say an African, but it is not clear whether or not he was actually born in Africa. He does say that he was a soldier since he was seven years old, and that he had...
Othello is a play, so you should not think of it as a book. Go and watch it, either at a theatre or on film, if you want to understand it. Learning how to understand plays by just reading the script...
Roderigo wanted to marry Desdemona (although neither she nor her father were much interested in his suit) and Othello made this impossible by marrying her himself.
When Othello finishes his monologue the duke relieves him of any guilt he may have for using black magic to marry Desdmona, and ultimately dismisses Desdmona's father's claims.