Senno di poi is how you would say hindsight in Italian. As in English, it refers to the ability to evaluate or comprehend an event, but only after it has happened.
Senno (masculine) means judgement / sense / wisdom, whereas poi is the word for then / later on / after, so the expression literally translates as after–wisdom.
Col senno di poi, saremmo potuti rimanere un giorno in più.
In hindsight / In retrospect, we could have stayed one extra day.
The proverb Del senno di poi (ne) son piene le fosse famously appears in chapter 24 of the Alessandro Manzoni novel I Promessi Sposi (The Betrothed). It suggests that if something unfortunate happens, it’s easy but pointless to say what should have been done in order to prevent it or to achieve a better outcome.
Del senno di poi (ne) son piene le fosse.
Literally: The pits are full of hindsight.
The proverb more or less equates to the English expression Hindsight is 20 / 20. That said, it might be easier for learners to say something along the lines of:
È facile parlare col senno di poi.
Literally: It’s easy to talk in hindsight.