What does Genesis 30:21 mean?

Scripture is often short on details, especially when those details are not especially relevant to the point at hand. This even includes genealogies and family trees, where the Bible frequently only mentions key figures. In other words, unless the Bible explicitly says someone did not have other children, it's possible that there may have been births which went unreported in the text. So far, the Bible has recorded the birth of ten sons of Jacob, and this is the first daughter of whom we're told. The sons will become patriarchs of the tribes of Israel, making their mention necessary.

A likely reason Dinah is mentioned, even if other daughters were not, is that she will become a key character in the story of Jacob's family later on (Genesis 34). Though we're not told why Leah gave her the name, the Hebrew Diynah means "judgment" or "vindication." Her birth was one more piece of evidence that God had vindicated Leah, in spite of the fact that her own husband seemed to value her so little (Genesis 29:31). This also reflects on the nature of the competition still boiling between Leah and her sister, Rachel, over their mutual husband, Jacob (Genesis 30:8).