"Supply" refers to products or services that are available for sale. Supply of goods is affected by the availability and price of materials needed to manufacture the products, the consumer interest in purchasing the goods, government regulations that may impact the manufacture or distribution of the goods, and so on. Supply is generally controlled by the seller - the manufacturing company making products to be sold or the supplier of services being offered to the public market.
"Demand" is the interest in purchasing the goods or services. If there is a high level of interest in buying a given item, that would mean it was in high demand. Demand may be affected by changes in market conditions, adjustments in pricing of products, and other factors. Demand is dependent upon the buyers in the market.
High demand sometimes results in temporary low supply situations, as when a store sells out of a popular item.