Bhashya
दध्योदनं दध्ना चरु पाचयित्वा; द्विवेदं चेदिच्छति पुत्रम्, तदा एवमशननियमः ॥
In plain words
'Curd-rice' is simply the oblation-rice cooked in curds. If it is a two-Veda son a man wants, then this is the diet he and his wife must keep.
Close to the Sanskrit
'Curd-rice' means the oblation-rice cooked with curds; if one desires a son [who will master] two Vedas, then this is the rule of eating [to be observed].