When the concert master (first chair violinist) comes on stage, it is their job to tune the orchestra. The concert master plays a concert A for the orchestra. The sounds you then hear are those of each instrument tuning to the A given by the concert master.
Since each string instrument (violin, viola, cello, and double bass) has four strings, they must tune each of these strings.
The winds and brass as well tune to the concert A. Each of these instruments is unique, so depending on its particularities, these instruments may or may not tune to more than just the concert A. As a clarinetist, for instance, I like to tune not only my concert A, but also the the octave A's above and below, as well as the fifth below (concert E-natural). This is because the clarinet's tuning acts differently in different registers of the instrument.
There can be variations in this tuning tradition when there is a soloist playing with the orchestra. For example, if there is a piano concerto being performed, usually the concert master will play an A on the piano instead of on his/her violin.