One entity occurrence does not require a corresponding entity occurrence in a relationship
This shows a zero or many. The many side is optional.
This can also be read as:
Right hand side: A customer can be in a minimum of 0 orders or a maximum of many orders. Left hand side: The order entity must contain a minimum of one related entity in the customer table and a maximum of 1 related entity.
This shows a zero or one. The 1 side is optional.