This verse (1 Chronicles 23:6) is essentially stating that King David, in his reorganization of the Temple services, organized the Levites into divisions based on their ancestral lines. The Levites were descendants of Levi, one of the twelve sons of Jacob (Israel), and they were specifically designated to perform religious duties in the Tabernacle and later the Temple.

Here's a breakdown:

"David divided them..." This indicates that King David initiated a systematic reorganization of the Levites to ensure the Temple services were carried out efficiently and effectively. This was part of his larger efforts to prepare for the construction of the Temple by his son Solomon.

"...into divisions..." This highlights that the Levites were not just a homogenous group, but were further categorized into smaller units for specific tasks and rotations of service.

"...according to the sons of Levi: Gershon, Kohath, and Merari." This is the crucial part. It explains that the divisions were based on the three primary family lines within the Levites. Levi had three sons, and these sons became the founding fathers of distinct clans or families of Levites:

Gershon (also called Gershom): Descendants of Gershon.
Kohath: Descendants of Kohath.
Merari: Descendants of Merari.

So, this verse essentially means that David organized the Levites into service groups based on their lineage, tracing back to the three sons of Levi: Gershon, Kohath, and Merari. Each of these families likely had specific roles and responsibilities in the Temple services. Subsequent verses in 1 Chronicles 23, 24, 25, and 26 detail the specific roles assigned to each family and their descendants, including gatekeepers, musicians, treasurers, and other positions. The division by ancestry made organizing the large number of levites easier, as specific skills and roles were passed down through families.