The verse "Yahweh spoke to Moses and to Aaron, saying," from Leviticus 15:1 is a common formula used throughout the book of Leviticus and other parts of the Pentateuch (the first five books of the Bible). It serves as an introduction to a new section of laws or instructions that Yahweh (the Hebrew name for God) is communicating to the Israelites.

Here's a breakdown of what it signifies:

Yahweh: This is the proper name of God in the Hebrew Bible. Its use emphasizes the divine authority and origin of the following commands. It highlights that the instructions are not merely human customs or traditions but come directly from God.
spoke: This indicates a direct and authoritative communication. God is actively addressing Moses and Aaron.
to Moses and to Aaron: This specifies who is receiving the communication. Moses and Aaron are the key leaders of the Israelites at this time. Moses is the primary lawgiver and prophet, and Aaron is the high priest. By addressing them both, God is ensuring that the instructions are disseminated to the people through the proper channels of religious and political authority. The verse does not automatically mean that both individuals heard exactly the same message or understood it identically. It highlights their unique responsibilities.
saying: This signals that the following verses will contain the specific content of Yahweh's communication. It prepares the reader for the detailed laws and regulations that are about to be presented.

In the context of Leviticus 15, this verse introduces laws concerning bodily discharges. The laws in this chapter deal with ritual impurity and the procedures for purification.

In short, the verse "Yahweh spoke to Moses and to Aaron, saying," establishes the divine origin, authority, and intended recipients of the laws that follow. It's a literary device that underscores the importance of the regulations as commands from God that are to be followed by the Israelites.