What do they mean when economists say that the demand for a product has decreased?

When a store owner or business person wants to graph their sales to determine which items are selling well and which ones are not selling well but staying on the shelves for long periods of time will create a demand curve.

This graph shows the connections between the cost of a specific items and the number of customers who are willing to buy this one item at any price. When you look at a demand curve, the line may be decreasing which shows that the quantity is decreasing.

That means that the quantity of that item is in need to restock the shelves. So, if the demand for that product has decreased, then the demand curve which shift to the left and it would look like it is increasing on the graph.