Supply refers to the quantity of a good or service that producers are willing and able to sell at different prices during a specific period. The supply curve shows the relationship between the price of a product and the quantity of the product that producers are willing to supply.

The supply curve is upward sloping because quantity demanded decreases when price increases. This can be explained by the following reasons:

1. Law of Supply: The law of supply states that as the price of a product increases, producers are willing to supply more of the product, and as the price decreases, producers are willing to supply less. This is because higher prices offer higher profits for producers, incentivizing them to increase production.

2. Production Costs: When the price of a product increases, producers may need to invest more resources and incur higher costs to increase production. For example, if the price of raw materials or labor increases, it becomes more expensive for producers to produce additional units of the product. As a result, they may reduce the quantity they are willing to supply.

3. Diminishing Marginal Returns: As production increases, producers may experience diminishing marginal returns. This means that each additional unit of production may require more resources and effort, leading to higher costs. As a result, producers may be less willing to supply additional units at higher prices.

4. Profit Maximization: Producers aim to maximize their profits. When the price of a product increases, producers can earn higher profits by supplying more units. However, as the price decreases, the profit margin per unit decreases, and producers may reduce the quantity supplied to maintain profitability.

In summary, the supply curve is upward sloping because quantity demanded decreases when the price increases. This is due to factors such as production costs, diminishing marginal returns, and the goal of profit maximization.