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 in which you may add metadata.
Add metadata to affiliates
Simply open your Affiliates Overview > click on the affiliate.
In the right-side panel, the option to add metadata will appear.
Add metadata to Conversions or Customers
Go to your Conversions page or Customers page > click on the conversion/customer.
Int he right-side details panel, you'll find the option to add and edit metadata.