Once there is a surplus of food what does that allow people to do

More generally speaking: it allows people to do other things than pursue food. This is very important because in the first years of humanity people spent almost all their time looking for food.

A surplus of food means people can get organised in bigger hierarchical groups, and that some people can devote themselves to other tasks, such as ruling, art, improving technology. This is what contributes to the development of civilizations.