Ficelle means ‘string’ and it’s generally the thinnest loaf you can buy in the boulangerie, short of a breadstick (gressin). The rules say they should weigh 200gr. They are often used for a jambon-beurre sandwich.
In some parts of France, however, they are shorter and stumpier.
Images by Monica Arellano-Ongpin, Nono64