As the protostar collects matter, it starts getting really hot. So even though it isn't burning hydrogren yet, it is glowing just from the heat. This can be hard to spot in visible light waves, especially since most protostars' light is blocked partially by the protoplanetary disk of dust surrounding them, as we can see here with the dark line in the center of the nebula.

The areas where the disk of thick dust is not blocking the light allow the nebula to be lit up by the protostar. Webb's infrared abilities help make this protostar's light more visible in the image, as most heat is emitted in the form of infrared wavelengths.