Graphene is not a perfect conductor of electricity. So when you put an electric current through graphene it heats up a little. Put a lot of current though and it gets very hot and also emits light just like the bars of an electric heater. This is called Joule heating.
The team made graphene get very hot, up to 3850K (3577 OC) using 2.5V and a current of 1.4mA.
The fascinating thing about this research is not that the team made graphene emit light by heating it. This has been done before. The clever thing about this is the way the team fabricated the graphene bridge over the electrodes.
This is the first time I have seen the possibility to manufacture graphene components from chemical vapour deposition graphene by a relatively straightforward process.