The Id field is the unique identifier for each entity in Enterprise Tester.
Identifiers for entities can be extract from the Url of edit screens, or better yet just drag the entity from the tree directly into the TQL query screen, at which point you will be provided with a set of suggested query fragments that can be inserted, which will include the ID:
Find the entity with the supplied unique identifier:
Id = 'a7c1c1fb-03a6-4318-859d-9f8200f6690f'
Entity, Package, Relationship, Run