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 by dragging the entity from the tree directly into the TQL query screen. You will be provided with a set of suggested query fragments that can be inserted and will include the ID:
Find the entity with the supplied unique identifier:
Id = 'a7c1c1fb-03a6-4318-859d-9f8200f6690f'
=, IN, !=, NOT IN
Entity, Package, Relationship, Run