So let's say you have 10 prod city building something that requires 100 prod to build and you slot a policy card to give +20% production to that item. Usually it would take 100/10 = 10 turns to complete. With the card, your city will get +20% production instead, so 12 production. 100/12 = 8.33 turns. It gets rounded up (at the end of the 8th turn it will have contributed 96 production with 4 left to do, so it will take another turn to complete), meaning it takes 9 turns to complete.
Hopefully that is clear and understandable how it works.