A lingua franca is a language that may not be someone’s native language, but it is a language that they know well enough to get by in. A language becomes a lingua franca due to economic and political forces, and has nothing to do with how good the language sounds to someone or the grammar of the language.
Some said that Latin was a lingua franca because of the preciseness of the language, but the Roman empire was probably more responsible. This video explains some of the forces that contribute to a language becoming a lingua franca.