Custom fields are a really useful and valuable way of making your assets easier to organise and find.
You can read about custom fields, with an example, in this article.
Once you've decided what field you'd like to create, you can do so as follows:
Option 1 - In the admin section
Go to Admin > Fields
Click New field
Choose from one of the example fields, or click Create custom field to make your own
Choose your field type
Depending on the type of field you chose, populate the field with your desired options
Option 2 - from the upload screen
Upload an asset
In the metadata sidebar, scroll down to the Configuration section
Choose from one of the example fields, or click New field to create your own