when we move along a given demand curve,

When we move along a given demand curve, it means there is a change in the quantity demanded of a good due to a change in its price, while all other factors affecting demand (such as income, tastes, and prices of related goods) remain constant. This movement can be either downward (an expansion or increase in quantity demanded when price falls) or upward (a contraction or decrease in quantity demanded when price rises). The demand curve itself does not shift during this movement; instead, we just move from one point to another on the same curve. This is distinct from a shift in the demand curve, which occurs when factors other than price change, causing the entire curve to shift right or left.