Stock Keeping Unit, otherwise known as a Part Number.
number of my product I have it?
If you wish; one product could have different SKUs though. If the 'unit' is something you want to be able to count and track separately in inventory then you need a SKU for it.
Say that you make a table. And you sell it in black and in white. If store the tables unpainted in inventory and you paint them only just before you ship them out to the customer then you only need one SKU because you are only tracking one thing in inventory.
If you however paint the tables as soon as they are built and you want to keep track of how many white tables you have and how many black tables then you need two SKUs, one for each color. The product id would probably the same for both because you need a way to track how many tables were sold without having to add up the sold numbers per color.
Be careful with the Invoices starter solution. It uses user editable primary keys. It can be dangerous.