It's from an old movie. It's about a Dysotopian Future where there are 40 Billion people on the Earth. Food is running out; luckily, the largest Food Company on the planet, and the only one left, has come up with a solution: Soylent Green. No matter how many people buy Soylent Green, it never runs out. People to continue to be fed. How do they achieve this miracle, you ask?
By digging up dead people and putting them into cans of food. Isn't that wonderful?
So yeah, now you'll understand whenever somebody references Soylent Green.