When using and then, the second expression is only evaluated after the first has evaluated to 'true'. That means that the field in the second expression will not be asked before the first one is answered.
This is a good operator to use when a condition only needs to be evaluated under certain circumstances.
e is Entity
e.entity_type = trust
e.number_of_trust_founders > 0
This composed condition could be attached to a document needed for trust founders. You only want to ask about the number of trust founders, if the document is actually about a trust.
That means you first determine the type of entity. This happens on line 2 of the example, where you want the system to check if the entity is a trust or not.
If it is, and only then, you want to ask how many founders the trust has. You wouldn't ask that, if the entity isn't a trust.
That's why and then is the operator to use here: only ask about the number of trust founders, after the first question on the entity type has been answered and evaluates to 'true'.