consumer is willing and able to buy at different possible prices, other things being constant

quantity demanded refers to the specific quantity or amount of good that the consumer is willing and able to purchase at a given price during a given period of time.

At zero price, demand is positive (shown by 'a'). The negative slope '-b' indicates an inverse relationship between price (Px) and demand (Dx). The minus sign shows that as price increases, demand decreases, and vice versa—highlighting the negative relationship between the independent (price) and dependent (demand) variables.

price of the product

fall in income - decrease demand - demand curve shift downward - at the given price

increase investment on infrastructure - increase in purchasing power

Increase in drought - decrease in purchasing power

the price of other goods - substitutes or compliment goods - affects the demand for the good - the shift upward or down ward as the case may be

the change in the taste and preferences will bring a shift in the demand curve

the current price does not matter, one's expectation for the future can also affect how much of a product one is willing and able to buy

as more or fewer consumer enter the market this has direct impact on amount of a product that consumer demand.