A line judge in tennis is an official whose first responsibility is to decide and communicate whether a ball landed in or out of bounds during a match. There are multiple judges, or umpires, each with different jobs and responsibilities.
Depending on the match, there can be up to nine line judges working during a single match. Each line judge is responsible for a specific line on the court, and has no say over any calls that involve other lines. Should the ball land on or near a judge's line, it is that line judge's responsibility to determine if the ball landed in or out of play.
Being a line judge can be a full-time job for some. Much like the athletes, many tennis officials have to work for years before working at major tournaments. They begin by officiating lower ranked matches and with experience and demonstrated skill, they can make it full-time working at the world's biggest events. 850 experienced umpires applied to work at the U.S. Open in 2019, and only 350 were selected. Those 350 are some of the finest line judges in the world, and those umpires can work full-time. However, most line judges are part-time workers.
Each line judge makes calls on the shots involving the line they are responsible for, but the chair umpire has final say over every call.