Supply affects demand and demand affects supply. What does this mean? Glad you asked.
If the supply (how much the store has) is high, then the demand is usually low. If the demand is high, then supply is usually low. In most cases, demand and supply adjust accordingly.
Here's an example:
The supply of a certain metal is low, and the demand is high.
The demand of a certain type of curtain is low, though the supply is high.
An example of when supply and demand evens out:
The demand for personal computers is high, and the supply is high. The supply used to be low when demand was high.
An example of supply and demand evens out to a low amount:
The demand of ink was high, and the supply was low. Today, the demand and supply is low.