If you're going boating this summer on those French rivers or canals run by the VNF (Voies Navigables de France) you'll need a vignette (label). It's like the old tax disc on a car, or a windshield regsitration sticker, and shows that you've paid for the privilege of cruising.
They're available for one day, seven days, a month or a year, and - as an economy of scale - are 'cheaper' the longer the period you select. They're also priced according to the length of your boat.
We just got ours for the next two weeks' cruising, but it was cheaper to buy a month vignette than two seven-day ones. The tariffs for 2018 are here, and you can purchase them online and print them out yourself.
You're supposed to display them in the window(s) of your boat so that lock-keepers and other officials can see them. In our experience they are very rarely checked, and if you happen to be cruising where you operate the locks yourself then there's nobody to check them anyway. But you never know when the waterways gendarmes may pay a visit!