Meanings
To blame someone for doing the same thing you are guilty off

Examples
Are you accusing me of spending too much? Look who's talking! We all know that when you enter a store you never ever come out empty handed
Senti chi parla! Mi accusi di spendere troppo? Proprio tu che quando entri in un negozio non esci mai, dico mai, a mani vuote

Everybody knows he led a hell of a life in his younger days and now he has the nerve to preach moderation and sobriety to today's youth! All I can say is: “Look who's talking!
Tutti sanno che da giovane ne ha combinate di tutti i colori, ma ai ragazzi d'oggi predica solo moderazione e sobrietà. Mi viene da dire: “Senti chi parla!”