[light][light] section defines lights of the keyboard, aka capslock, scrolllock and numslock. They are actually output pins, so there are two fields available: pin and low_active.
pin field is just like IO pins in [matrix], and low_active defines whether the light is low-active or high-active (true means low-active).
You can safely ignore any of them, or the whole [light] section if you don't need them.
To use [light], you need enable controller feature in your Cargo.toml