Metadata can be used by very select use cases - usually you'd retrieve this metadata with a conversion, or add it via the API. However, we've also introduced the option to add metadata through the interface.
Below you'll find the places where you may add metadata.
Add metadata to affiliates
Simply open your Affiliates Overview > click on the affiliate from the list.
In the sidebar on the right, the option to add metadata will appear.
Add metadata to Conversions or Customers
In the sidebar on the right, you'll find the option to add and edit metadata.