What is RER? IS Europass can it be use on any kind of train in Europe?

We are travelling to France for 13 days. But also we would like to go to different places within France such as Strasbourg, Bordeaux,Lyon, Mont St Michel,Avignon & of course Paris. But we would like to squeeze thr Rome for 4 days. Please let me know which train I should take & which passes I should get?

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