In this query, we retrieve the node ids of all the published nodes that have been last updated before the current time, that have the word cat inside their title and that have a taxonomy term called cats as a reference in the field_tags. As you can see, there is no more distinction between propertyCondition and fieldCondition (as there is in D7 with EntityFieldQuery). Additionally, we can include conditions based on referenced entities tacking on the entity.(column) to the entity reference field name.