After the salat finishes it is sunnah to recite Istigfar 3 times. The reason for this is that obviously during salat we might get thoughts about something else. The Istigfar is there to rectify any shortcoming in our salat.

Translation for "Astagfirallah" is "I seek forgiveness from Allah".

To say Istigfar 3 times after salat is not some cultural tradition but actually from the sunnah of the prophet (peace be upon him)