You can julienne any vegetable, but in general a julienne de légumes means a mix of shredded carrots, courgettes, and celeriac, often used as a base for salads. You can buy bags of frozen ones in the supermarket (these may include red bellpeppers, cauliflower or cucumber etc). They are often wrapped up with fish en papillote.
Preparing them at home can be tedious, but there’s a special tool for julienning called a rasoir à julienne that helps. Many mandolins also have a julienne attachment as well.
Images by Coyau / Wikimedia Commons / CC BY-SA 3.0, Frédérique Voisin-Demery