This is a concept that is associated with testing of terminals in a specific situation, as to whether they have similar levels of instantaneous polarity, where it is assumed that terminals are not marked.
When a high and a low terminal are joined together, as a mandatory issue voltmeter is placed within the remaining terminals. A moderate value is placed on this concerned terminal.
In a transformer, there are 2 terminals and each of the primary as well as secondary terminals become positive and negative in an alternate manner. On a singular basis, polarity in a transformer has no value. However, when 2 transformers are placed together, it is then that this concept of polarity is introduced, where 2 terminals which have the same instantaneous polarity have to be connected with each other.
This is obtained by leaving out one high voltage and another low voltage terminals and finally summing up values from both these points.