A concertmaster is typically the first chair first violinist in an orchestra. For performances, they typically do not come onto stage until the concert is ready to begin. They come onto stage (usually with applause) and lead the orchestra into tuning. It is the concertmaster, along with the first chair cellist, who leads the orchestra into standing in respect for the conductor when he walks on stage. The concertmaster is sometimes given a solo part in the music being played.
The concertmaster plays a much bigger role in smaller groups where a conductor is not needed. They act as the conductor, leading the ensemble and setting the beat. In larger orchestras, the role of the concertmaster is usually simply a reward for exceptional performance in the conductor's eyes.