The doctrine of the Oral Law from Sinai is one of the essential theological principles of Orthodox Judaism. The basic idea is that in addition to the written Torah (Pentateuch) handed down by God to Moses at Mt. Sinai, God also conveyed the oral Torah, that explained in detail how the 613 laws of the Torah were to be observed in practice. Moses handed down this information to Joshua, who transmitted it to the Prophets, who conveyed it to the Elders, then to the Sages of the Talmud in an unbroken chain until today.
The effect of this doctrine is to invest the legal rulings of the ancient rabbis and their modern counterparts with divine authority. Rather than regarding the Torah’s laws as being the product of customs and traditions that evolved over many centuries as social conditions changed, they are regarded as infallible and thus immutable. This notion largely accounts for the stasis that characterizes the ultra-Orthodox worldview.
I discuss this subject in my book, but have also videotaped a brief, four minute summary, linked to here: https://youtu.be/kMN5aG_ygvw?si=XhSUbsAytf9Jri2M
