Seniority refers to being ranked higher in an organization since you have been there longer. Who has seniority in a high school? The seniors of course!
You know how older people are called senior citizens or seniors? That should help you remember what seniority is: the longer someone stays within an organization, the more they have of it. A new worker has hardly any seniority. Someone working at a job for 20 years has a lot of seniority. Having seniority is like having a higher rank or status.