Use the ID field to automatically add an unique id to every card. You can use this id to reference a specific card from other systems or to reference a card in for example an e-mail message.
The ID field has an optional prefix which you can use to make it easier to identify what kind of card a specific id references.
For example: setting the prefix field to "PROJECT" adds an unique id in the like "PROJECT-10001" id to each project card.
You can easily find a card based on the id by pasting the id in the searchbox directly in the top toolbar.