When an orchestra gives a concert the instruments have to tune carefully so that they are in tune with one another. Usually it is the principal oboist’s job to stand up and play the note A so that everyone can tune to that note. In some countries such as the US it is tradition for the principal violinist (concertmaster) to give the A. If the orchestra are going to play with a piano soloist they will have to tune to the A of the piano because the piano has already been tuned by the piano tuner.