According to Exodus 24, Moses sprinkled blood on the Israelites at the covenant ceremony. He built an altar, arranged twelve pillars for the twelve tribes, and offered burnt offerings and young bulls as fellowship offerings. Moses then took some of the blood and sprinkled it on the people.
Before the sprinkling, he read the Book of the Covenant aloud, and the people promised to obey everything the Lord had spoken. The blood therefore marked the formal ratification of the covenant between God and Israel at Mount Sinai. Exodus records Moses declaring it the blood of the covenant.
This ceremony should not be confused with the later rituals of the Tabernacle or with the blood placed on doorposts during the first Passover. Those events involve different settings and purposes. In Exodus 24, the blood specifically accompanies the Sinai covenant ceremony after the people receive the covenant words.