The Concertmaster (American English) or leader (British English) is the most important violinist in an orchestra. He or she will sit in the front seat, by the conductor's left. The word concertmaster comes from the German Konzertmeister.
The leader will be the highest paid member of the orchestra. He has to decide how the violins will play the music and write in the bowing. He will decide where each member of the violins should sit. He will probably also interview them and offer them the job in the orchestra. He will talk to the conductor about anything the orchestra are not happy about (he may have to remind the conductor when it is time to stop the rehearsal!). If part of the music is marked "solo" then he will play it as a solo while the other violins stop playing.
In the United States it is usual for the concertmaster to be on the platform before the concert and to tell the orchestra to tune their instruments, playing on his A string to give them the correct pitch. In European orchestras it is usually the oboe that gives an A for tuning. In Britain the leader usually comes on stage after the orchestra have tuned, and gets an applause.