total surplus
Picture buying a concert ticket. You would have paid up to 100 dollars, but it cost only 60 — you walk away feeling 40 dollars 'ahead'. Now picture the seller, who would have accepted as little as 40 but received 60 — they too come out 20 dollars 'ahead'. Both sides gained from the same trade. Total surplus is simply the sum of all such gains, for every buyer and every seller in a market, added together.
Formally, total surplus = consumer surplus + producer surplus. Consumer surplus is the gap between what buyers were willing to pay and what they actually paid; producer surplus is the gap between the price sellers received and the lowest price they would have accepted (roughly their cost). In the ticket example, total surplus from that one trade is 40 + 20 = 60 dollars of value created out of thin air, just by letting two willing parties trade. Sum this over the whole market and you get a money-measure of the total benefit the market generates.
Total surplus is the central scorecard of welfare economics: an outcome is 'efficient' when it is as large as it can be, and a deadweight loss is the chunk of surplus that vanishes when a market produces too little or too much. It is powerful but limited — it counts a dollar of gain equally no matter who receives it, ignores effects on bystanders unless we add them in, and only captures benefits that can be expressed in money. So a bigger total surplus means more wealth created, not necessarily a fairer or happier society.
At a farmers' market, ten shoppers each value a basket of apples at different amounts above the 5-dollar price, and the seller's cost is below 5. Add up every shopper's bargain plus the seller's margin, and that sum is the total surplus the apple market created that day.
Gains to buyers plus gains to sellers, added up.
Total surplus counts every dollar of gain the same regardless of who receives it, so 'maximizing surplus' is about the size of the pie, never how it is sliced.