Custom Fields allow you to create your own fields and then fill them out with individual values for each product.
You can then assign the Fields within the template for the item specifics e.g. Size and it will then take the size from the product, instead of from the listing template; This makes it much easier because you will not have to change the item specifics on the listing template each time for every product.
Click on Configuration.
Click on Product Settings
Click on Manage Custom Fields then the Add Custom Field button on the right hand side of the screen.
Name the Field and choose the type from the drop down box Select you can also set a Default Value if it's applicable
Congratulations you have successfully added a Custom Field.