It is the anchor for the Etnachta group, which in full consists of four different trope sounds, not all of which are always present.
These are Mercha, Tipcha, Munach, and its namesake Etnachta.
The Etnachta group marks the end of the first segment of a verse.
An example is in the first verse of the Book of Genesis according to the Masoretic Text, the statement that God created is marked with an Etnachta, showing the completion of God’s creation.
[2] The Hebrew word אֶתְנַחְתָּא translates into English as pause.