It is a language used to communicate among non-native speakers.
English has become the most widely used Lingua Franca in the world, hasn´t it? For example: a Frenchman and a German that meet in South Africa will probably communicate in English to each other, especially if neither of them speak the other's language.
Do you know any other Lingua Francas? Watch this very interesting video and learn about Esperanto, a language that was created to change and improve the world in a time full of idealism: