You have to decide how to allocate your budget money among all the things you need or want. The higher the price of any one thing you want, the more other things you give up when you buy it, because you have less money left over. Higher price items have a greater opportunity cost for you. So you buy less of something when its price is higher. The higher the price is, the easier it is to make a profit selling in this market.
High prices attract more sellers and induce an expansion of production.
The price at which the quantity supplied equals the quantity demanded is the equilibrium price. The corresponding quantity is the equilibrium quantity. The equilibrium is where supply and demand cross. 20 is the equilibrium quantity. At prices below equilibrium, there is excess demand.