This phrase kadhalik Allahu Rabbi کَذَلِکَ اللّٰهُ ربّي) means Indeed this is how my Lord is. Such an expression is said after the reciting of Surah al-Ikhlas, because of how profoundly lofty the words and meanings of this small but very important surah is.
So, when you recite this surah, you affirm, acknowledge and confess to what you just said by saying kadhalik Allahu Rabbi.
According to our narrations from Ahlul Bayt (a.s.), saying this phrase after Surah al-Tawhid is mustahab, and it can be said once, twice, or three times. The phrase كذلك الله ربنا has also been mentioned as well.
And Allah knows best.