This verse describes a specific duty assigned to some of the Levites, specifically from the Kohathite clan. Let's break it down:

"Some of their brothers, of the sons of the Kohathites": This identifies the people performing the duty. The Kohathites were one of the major divisions within the tribe of Levi. The Levites were responsible for the temple service.
"were over the show bread": This refers to the "showbread" or "bread of the Presence." This was a sacred offering of twelve loaves of bread that were placed on a special table in the Holy Place of the Tabernacle (and later the Temple). It symbolized God's provision for His people and their continuous dependence on Him.
"to prepare it every Sabbath": This indicates that this was a regular, weekly task. The showbread had to be freshly baked and presented every Sabbath (Saturday), and the old bread was then eaten by the priests.

In summary, the verse means that a specific group of Kohathite Levites were responsible for baking and preparing the showbread each Sabbath, ensuring that this important offering was always ready for presentation in the Tabernacle/Temple.

Significance:

Division of Labor: This verse highlights the intricate division of labor within the Levitical priesthood. Different groups had specific responsibilities to ensure the smooth functioning of the Tabernacle/Temple service.
Importance of Ritual: The careful preparation of the showbread shows the importance placed on ritual and proper execution of religious duties.
Levitical Duty: It underscores the role of the Levites in maintaining the religious life of Israel, acting as intermediaries between God and the people.
Continuity: The recurring nature of the task ("every Sabbath") emphasizes the consistent and ongoing nature of worship and dedication to God.