Nobody sets the price of coffee. No committee meets to decide it. Yet a price exists, it changes, and it somehow rations a limited supply among everyone who wants some. Understanding how that happens without anyone arranging it is the single most useful thing economics teaches, and it starts with two curves.
Demand
The quantity of a good consumers are willing and able to buy at a given price over a given period.
Supply
The quantity of a good producers are willing and able to sell at a given price over a given period.
The demand curve slopes downward because as price falls, buyers who were previously priced out enter the market, and existing buyers find the good more attractive relative to substitutes. The supply curve slopes upward because higher prices make production more profitable, so firms devote more resources to that good and higher-cost producers find it worth entering.
The phrase 'willing and able' is doing real work and examiners look for it. Wanting a car is not demand. Wanting a car and having the money to buy one is. Candidates who write only 'how much people want to buy' rarely secure the second mark.
Shifts versus movements
This distinction separates strong answers from weak ones, and it comes down to a single question: what changed? If the price of the good itself changed, you move along the existing curve, and that is called a change in quantity demanded or quantity supplied. If anything else changed, the whole curve shifts, and that is called a change in demand or supply. Income, tastes, the price of substitutes and complements, and population all shift demand. Production costs, technology, taxes, subsidies, weather and the number of firms all shift supply.
The most common diagram error in this topic is shifting a curve when the question describes a price change caused by the other curve moving. If the price of coffee rises because a frost destroyed the harvest, supply shifts left and you move along the demand curve. Demand itself does not shift. Ask yourself what caused the price change before you draw anything.
The government raises income tax, reducing household disposable income. What happens in the market for restaurant meals, a normal good?
Show the effect on the market for wheat of a government subsidy paid to farmers.
Supply shifts right. Mark both the original and new equilibrium, label the axes, and show the direction of change in price and quantity.
Equilibrium price
The price at which quantity demanded equals quantity supplied, so there is no tendency to change.
Away from equilibrium the market corrects itself. Above the equilibrium price, supply exceeds demand and the resulting surplus pushes sellers to cut prices. Below it, demand exceeds supply and the shortage lets sellers raise prices. Neither adjustment requires anyone to plan it, which is the central insight of the price mechanism.