Custom Attributes provide a way to add non-standard details or information about a resource (object) in the Pinpoint system. These can then be displayed or used for custom logic (e.g., conditionals, filtering, reporting).
Each Custom Attribute must be associated with a Custom Field. You can think of this as pairing labels/keys (Custom Field) with values (Custom Attribute). A Custom Attribute has a value which type is defined by the Custom Field which it's linked to (custom_field.field_type).
You can find the current list of supported objects for which you can have Custom Attributes in the Create Custom Attribute endpoint documentation.