What does @Transient annotation mean for methods?

All field-level JPA annotations can be placed either on fields or on properties, it determines access type of the entity (i.e. how JPA provider will access fields of that entity - directly or using getters/setters).

Default access type is determined by placement of @Id annotation, and it should be consistent for all fields of the entity (or hiererchy of inherited entities), unless explicitly overriden by @Access for some fields.

So, @Transient on getters has the same meaning as @Transient on fields - if default access type for your entity is property access, you need to annotate all getters that don't correspond to persistent properties with @Transient.

Well it's a proper getter method, which JPA by default will assume is bound to an entity property. If you don't want JPA to treat a getter as a property, you apply the @Transient annotation to the method.