I am the LORD … God Almighty: Here, the Hebrew El-Shaddai is translated “God Almighty.”
God was emphasizing that he was revealing himself in a new way. The patriarchs (Abraham, Isaac, and Jacob) knew him especially as God Almighty. Now he was making his name Yahweh (LORD) known more fully to Moses and the Israelites.
The point is not that the name Yahweh was completely unknown before. Rather, God was now showing more clearly what that name means through his powerful acts of rescue.
by My name the LORD I did not make Myself known to them: The name LORD (Yahweh) appears often in Genesis. There are two possible explanations:
The patriarchs did not know the name Yahweh, but God inspired Moses, who wrote Genesis, to use it. This was wherever God's grace and covenant-keeping nature were evident.
While the patriarchs might have known the name Yahweh, they might not have seen God's nature as clearly as during the exodus and the Sinai covenant.
In Hebrew, a person's name has more importance than in English. Names showed character and nature, and were not only labels (see, for example, Psalms 8:1, 9; 148:13).
Here, make known implies intimate knowledge and experience. In this case, the patriarchs knew God’s name, but they did not know and experience his nature fully as he revealed himself in the exodus.