Q: What does sodium citrate do in food?
partial A: Sodium acts as the Na+ of plain NaCl - important extracellular ions. Na+ contributes for example to the depolarization of neural cells and heart (muscle/conductive) cells. So, you need it to think and stay alive. Generally not dangerous, but quite a large percentage of the afro-american population is more than average sensitive to high Na+ load (raises blood pressure in these individuals). But in general it's very well regulated in the body, with minimal fluctuations.
Citrate is the "first" molecule of the citric acid cycle, which produces energy in the form of ATP. The citric acid cycle needs oxygen: it's aerobic. Important for almost all cells, excluding mainly red blood cells. Citrate is formed from oxaloacetate and acetyl CoA, the latter being the last stage of the (anaerobic) glycolytic pathway. Citrate is also important as a rate-limiter of glycolytic enzymes, and an initiator of fatty acid synthesis (also connected to the citric acid cycle).