What a trail shift is
A trail — also called a stage, a working interview, or a tryout — is a shift you spend shadowing and helping the staff, so the venue can see what you are actually like in the room. It is standard practice across the industry and in a lot of houses it is the real interview. The conversation beforehand was the screening.
It typically runs a few hours to a full shift. Whether it is paid varies by venue and by jurisdiction, and it is a fair thing to ask about directly when it is offered:
"Happy to come in — is the trail paid, and how long should I plan for?"
Ask it plainly and without apology. It is a normal question, the answer affects your planning, and being unwilling to ask it is a habit that costs money later in this job.