Brains are delicate things…
Profit and Surplus
The surplus is what’s “left over” after work has been put in (to a field, a factory etc.) and a product created (a crop, a shirt etc.). In modern times there is a huge carbon subsidy- solar energy that was converted into fossil fuels millions of years ago burned to boost human activity.
If that surplus is kept in private hands it is profit, often spent on prestige and pleasure (huge mansions etc.) when it could have built schools, hospitals, or even better taught people how to teach, or public health. Or paid for some of the urgent ecological repair.
See Marx’s critique of the Gotha Programme. Surplus is necessary for “replacement of the means of production used up, for expansion of production, reserve or insurance funds to provide against accidents” etc.