A hot solid emits light with a continuous spectrum, i.e. there is no gap in the spectrum, every wavelength is present. A hot gas on the other hand emits light with a discrete spectrum, i.e. only certain sharply defined wavelengths are present.
In this comic, the boy is struggling to play the correct notes because the violin is a “continuous spectrum instrument”. By pressing along the violin string, one can produce sounds of any pitch, including off tune notes. A piano on the other hand is a “discrete spectrum instrument”. It can only produce notes that the piano keys have been tuned to, and nothing in between. Haha.