The change in demand happens when factors like income, related goods’ price, taste and preferences, or expectations of consumers change with time. The price of the good remains the same, but these determinants change that influence the demand behavior of consumers. These changes shift the demand curve.
For example, if the income of the consumer increases from $2000 to $4000, the consumer will increase the demand for normal goods. This shifts the demand curve forward, increasing the demand at each price.
Similarly, if a consumer expects that the price of a dress will fall from $10 to $5 in the future, he/she will reduce the current demand to buy the dress in the future. This will shift the demand curve backward.