A sequence drawing, is one way of showing how the product you are designing could be manufactured in industry (in a factory). Basically it is a drawing of each stage of manufacture and includes brief notes or instructions. It normally follows a flowchart which simply lists each stage.
A sequence drawing is an important way of working out every stage of making a product and it allows the designer to recognise any possible problems before the product is made. It may be possible to work out the cost of manufacture, the number of people needed on the production line, the amount of materials required and many other important factors - through the construction of the sequence drawing.
Remember, flowcharts and time charts are also important ways of planning a project. They are usually drawn/written first.