Here is a short blog about rail ticket validation in France. This is a very simple process of placing your rail ticket into the small yellow SNCF validation machines at all the major French railway stations, prior to your journey.
On most TGV routes it is required that you validate your ticket, prior to travel by placing your ticket in the grey mouth-like slot near the bottom of the machine. It allows you to insert one end of the ticket (length wise), use the left end of the ticket, where there is usually a bar code and an SNCF logo. Insert the ticket face up (magnetic stripe down). After you hear a printing noise similar to an old dot-matrix printer remove the ticket.
The validation machine has then validated your ticket so you are now ready to make your rail adventure!