Sorry if this sounds stupid but I don't get it.
From what I understand this is what they mean....
If you want to grow veg to eat, the then resources required to grow them, has an "environmental cost" for the whole process (water/soil/pesticides/electricity/co emissions from farming equipment/etc).
Imagine if you get 5 points of energy by eating veg, and the enviro cost to grow them is 10, that's a profit/cost ratio of 5:10 (or 1:2)
Say you want to raise animals to eat, then those animals also have an enviro cost too, because you need to "feed" those animals regularly. That means growing a lot more veg then what humans need/eat - just to feed the animals - that you're going to kill anyway.
Imagine if you get 12 points of energy out of eating meat, but the enviro cost to grow them is 40, that's a profit/cost ratio of 12:40 (or 3:10)
I made up the numbers for the example but that's basically why they say eating veggies (rather than raising animals just to eat them) is better for the environment. In summary; if you want to play a small part in saving the planet, then give up meat and be a veggie, but I'm still going to enjoy chicken wings with hot-sauce and fries