There is a composition relationship (the filled diamond) between Employer and Position. It indicates that the Employer owns or manages the Position, and the Position should not exist without the Employer.

There is an aggregation relationship (the empty diamond) from the Employer to its employees. The Employer groups together a collection of Persons and gets them to do things during working hours. In an aggregation relationship, the lifetimes of the objects on either end are not related to one other.

The Person and the Position have a bidirectional assocation between them. An association is a bidirectional relationship, and does not specify anything about implementation, ownership, or management between objects. It is possible that there is no actual pointer in Person that points to its Employer because it can be calculated by going to its Position and calling Position::getEmployer(). Although the diagram indicates a 1:1 relationship between Person and Position, it is possible that a Person can hold multiple Positions, or an Employer may hire many Persons for the same Position. If you want to describe a system that can handle such cases, then the cardinality of this association should be many-to-many.