I believe that literally means your character is glowing as mentioned by @luke-turbulenz in the below comment:
Edit: Pasting only the part mentioning the lightsource skill.
Also, there is a skill which will make your character a torch so you’ll be able to light up the area without holding a torch.
Source: I don't like it yet - #36 by luke-turbulenz
Honestly, I felt that it’s a bit wonky as a skill. It could be an equipment like helmet with flashlight or if we’re sticking to skill, could be for a certain race only. One which would make sense that his body is glowing. It really looks weird to see a glowing cat…