A trail is an interview in a restaurant. When you go into an interview, you are said to be trailing or "the trail". For a trail, you should bring everything you need to work on the line because you will likely be working service attached to a station unless you were told otherwise. Trails generally begin right after the cooks arrive and after you are shown where to change and where to find towels, an apron, and water, you can expect to be assigned to someone during prep and potentially someone else during service.
What Should I bring on a trail?
You should definitely bring a sharpie, a pen, a hat, slip resistant shoes, and long preferably black pants. I would never show up to a job cooking without those things. You should also bring a chef's knife. If you don't have one and you're looking for your first job, don't let that stop you from going on the trail. If the kitchen needs people, they will give you the tools you need to do your job. Beyond a chef's knife, maybe a paring knife, bread knife, and peeler would be useful, but not mandatory. It's probably best not to show up with too many gadgets.
Expediting chef during preshift: "Everyone say hello to Yasmine, she is trailing with us tonight. She's attending CIA and is looking for a place to extern, so I want for everyone to put up lots of tasters for her to try out and if you have side projects that need doing, don't hesitate to throw something her way."