Supply refers to the amount of a product that a producer is willing to make. Demand refers to the amount of a product that consumers are willing to purchase. Shifts in supply and demand affect both the quantity of goods sold and the price at which they are sold.
When supply and demand simultaneously increase, the quantity of goods sold naturally goes up. Consumers are willing to purchase more products, and producers are willing to make more products, so the market expands and more products are available and sold.
Price variations, however, are a bit more tricky when supply and demand rise at the same time. If the two rise fairly equally, the price of goods will remain about the same. If, however, the demand increases more than the supply (even though they are both going up), producers know that they can charge more for their products. The demand still outstrips the supply. If, on the other hand, supply increases more than demand (with both still going up), there will be excess goods on the market and prices will drop.
Let's look at an example. A producer creates and markets a particular computer. Predicting that demand will be heavy during the back-to-school season, the company increases production, which also increases the supply of computers. Demand does indeed rise, as students are looking to purchase new computers before the start of the school year.
In this situation, more computers will be sold overall, but three things might happen with regard to price. If the supply and demand for the computer rise equality, the company will keep the price about the same. There is an equilibrium, and it doesn't want to disturb that by raising the price. If the demand for the computer soars as happy students recommend it to others, the company will raise the price, for students want more computers than are being produced and the computers on the market are, therefore, more valuable. If, however, students fail to demand as many computers as the company has made, the company will drop its price to get rid of its excess stock.