What went wrong with this shot D5100 with f/1.8 lens

Reflective light meters, as in cameras whether set to matrix, center-weighted or spot, presume a scene that approximates to mid grey (18% grey card). For most photographs this is fine as most scenes contain a range of tones.  The problem arises when the scene to be photographed is excessively dark or excessively light ie contains a lot of black or a lot of white.  In the the scene of the OP there is a large black back drop, matrix might have evened out the exposure between the white shirts and the black back drop but it looks like the narrow "spot" metered a dark area and presuming mid-grey opened up the exposure to render the black grey and hence blowing the highlights or whites for these were now overexposed.

The camera does not apply exposure compensation it is just the way it meters the scene. The answer is to apply exposure compensation "-" for blacks "+" for whites either by changing the aperture or the shutter speed. Changing ISO does not apply exposure compensation it just changes the sensitivity of the sensor.  It might appear that is does for the exposure will change but the light on the subject is exactly the same. The amount exposure compensation depends on the amount of dark or light in the scene.  A snow covered landscape in bright sunlight may need as much as 2 stops exposure compensations. At least with digital you have an instant display and histogram to help you judge. If in doubt bracket.

In camera metering has come a long way from when it was first introduced but it still needs to be used intelligently by reading the scene and applying adjustment accordingly.  80-90% of shots will probably meter effectively with matrix.  You can of course buy 18% grey cards to meter the light falling on you subject and then use those set manually on your camera.

I hope that helps.