LOB field fetches your data directly from an external data source, such as database or web service. You specify the connection and the fields you want to extract and the field will do the rest.
You can select from a dropdown of possible items or enter manually. The imported item can contain as many additional fields as you like and you can even give simpler display names to those fields.
When the item changes in the LOB system, your SharePoint data is not affected, as long as you do not initiate a data fetch. This way you can be sure your SharePoint items stored the snapshot relevant to the point in time when your SharePoint item was created.