English Earnings from Polish middles, just as from ordinary middles, come from the event outcome «landing» in both (and sometimes more than two) oppositely directed outcomes.
Example of a simple middle: there are two bets: Over 0,5, odds 2,05, and Under 1,5, odds 2,50.
If exactly one goal is scored in the match in question, this outcome will fall under both outcomes, which means that the player who placed a bet on them will, as it were, receive their winnings twice. This is what landing in a middle means.
In the case of Polish middles, the calculation method is somewhat more complicated (it is described in detail on the page describing Polish middles).