Double stopping
On instruments such as the violin the player is mostly playing one note at a time.  It is not difficult to play two open strings at once, but fingering two strings at once can be very difficult, so double stopping is an advanced technique.  Sometimes triple or quadruple stopping is needed.  This is sometimes called multiple stopping.  It is not really possible to play three or four notes at the same time on a violin.  The bridge is curved so that the player can bow on one string without hitting the others by accident.  Multiple stopping is done by playing first two strings and then, while still letting the sound from these two ring, quickly playing the other two strings.