Hebrew, for “the descent (decline) of the generations.”
God perfectly taught Moses on Mt. Sinai. But we all participate in our first parents’ fallen nature Rom 3:23. The generation after Moses introduced tiny errors. The generation after that passed along these tiny errors as well as new tiny errors of their own. As each generation of men who participated in our fallen nature taught it to their children, the tiny errors accumulated over time. These tiny errors collectively are called yeridat hadorot, the decline of the generations.
Early sages who lived closer in time to Moses were, objectively speaking, less exposed to yeridat hadorot and therefore more reliable than later ones.
The root is yerida, going down, often used in emigration from Israel. Its opposite is aliyah. A Jew who moves to Israel is said to “make aliyah,” go up.
The same is true in the Catholic tradition. The Church Fathers who lived closer in time to Rabbi Yeshua’s shlikhim are considered more reliable than later Fathers. The time interval between the original apostolic teaching and a particular Father is, of course, only one of many factors in that Father‘s reliability. The dominant factor is God’s grace. Some later Church Fathers, such as St. Augustine, are extremely reliable.