You can find examples of this in the Invoices starter solution that comes with your copy of Filemaker. It's also used in this much simpler invoicing demo file: http://fmforums.com/forum/showpost.php?post/309136/
In both cases, you establish a relationship based on Product Number between the table where you enter this data (line items in the two examples I've mentioned.) and a table of products where you have entered the product Number and description for each product.
With that relationship, you can 1) use a field with the looked up value auto-enter option to copy the description from the related Products Record into a matching field in the line items record. 2) You can also just place the description field from Products on your layout.
Both methods have their uses as what happens when you edit a description in Products will produce different results in these two different methods. A looked up value field will not automatically update to show the changes from the edit. Since the other method uses the actual description field from products, the changes will automatically appear in that case.