Browse node ID are numeric codes that identify inside Amazon a specific sub category. You need to specify a node ID if you publish a product on Amazon without a barcode.
If your product has a barcode, the node ID becomes irrelevant.
In Amazon’s own words:
Browse node IDs are positive integers that uniquely identify product sets, such as Literature & Fiction: (17), Medicine: (13996), Mystery & Thrillers: (18), Nonfiction: (53), Outdoors & Nature: (290060). Amazon uses thousands of browse node IDs..
While top-level browse node values tend to remain the same, the values of other browse nodes change often. Browse nodes are created and eliminated without notification.
Please note that Node IDs are unique in one Amazon locale (e.g. Amazon.com, Amazon.co.uk, Amazon.fr, ... The same node ID can be used in two or more Amazon locales, but the names and purposes of those nodes can be different and unrelated.
If you are a pro seller, you can find here the node ID’s for each category in Amazon European Locales:
And so on.
Where to find an Amazon category Node ID.