From the ORM API Docs:
value:
variable type, must be comparable (through operator) to the named field
Either the documentation is vague or I'm particularly dense today. What does this mean?
field_name is the actual field of record which be compared to filter the records, value is the entity with which the records field_name will be compared with. Let's say domain=[('partner_id','=','1')], will check the field partner_id to be equal to 1 and filter the model records accordingly.