Girsa dyankuta means the things one learns with ones mothers milk. This is the literal translation. Girsa comes from the Aramaic, meaning to read or learn, especially by heart. Yankuta is a child who is nursing, from yonek, the Hebrew word which means to suckle.
The Talmud tells a story on the value of girsa dyankuta (Shabbat 21b).The sage Abbaye learns a halachah and is saddened that he did not know it before. Why the Talmud asks? He knows it now. Because it would never be part of his girsa dyankuta.
There is a deep, ineradicable quality to many of the things we learn when we are very small that later knowledge cannot match. Be careful what you teach small children; they will remember it!
Rabbi Adin Steinsaltz points out an unusual fact about traditional Jewish culture. The same core books are learned by small children and great scholars. It is not as if children are learning nonsensical nursery rhymes while the adults are reading Shakespeare.
Both study the Chumash and the weekly parashah, but on very different levels of understanding. The girsa dyankuta acquired when we are children can serve as a basis for lifelong learning.
Of course, sometimes the challenge is to break with ones girsa dyankuta, our childish understandings of religious ideas and texts, to make room for a grown-up grasp of those same concepts.