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Explanation: As the first proposed answer explains, this is not an underground/subway-specific term. It applies to any form of transportation. Aller=go; Retour-return so if you want to go somewhere and back on the same ticket you ask for a "billet aller-retour" or round-trip ticket.
Best Regards, DBG
Explanation: It simply means "return" (although literally it means there and back). It is not just Metro specific. This terms applies to any means of transport, trains, buses, coaches, airplanes...
Metro in British English = Underground
Metro in US English = subwy. (Subway in GB English means underground passage to get from one side of a busy rouad to the other or to cross under railway tracks). Although I am English, when I lived and worked in london, I discovered this small but very real difference when American visitors would ask for the subway. They were looking for the Underground and would sometimes be surprised to find themselves in a tunnel which just took them to the other side of the road!
RATP run the Paris Metro and their website is bi-lingual. (They use "subway" to describe the Metro).