It's all relative - but the far end of going light is backpacking style light. No cooler or ice. No cases of beer. 50-60 lbs of clothing, food, water, medical, and personal items. Add in a boat, minimal frame, repair kit, and required items (like a firepan (perhaps a oil pan), groover (waag bags and a plastic bucket, etc.) For me on a multi day that might look like:
Raft - 150
Frame, Oars - 55
Personal Gear - 75 lbs (includes river clothes, PFD)
Repair kit, pump - 10
Food, water (mostly pumped) - 25
required gear - 60 lbs
total min weight: 345 plus myself (200 lbs) equals 575 ish
You could also define "going light" as staying below a certain percentage of your crafts carrying potential. Say less than 1/3 of max load. So if my raft has a max load of 1800 lbs then under 600 lbs would be "going light."
When I think of paddle raft guiding I always thought that 2-3 people plus myself was a light load. That's 500-700 lbs in the boat depending on the people.