This verse, "On the third day Eliab the son of Helon, prince of the children of Zebulun," is part of a lengthy passage in Numbers 7 that describes the offerings brought by the leaders of the twelve tribes of Israel to dedicate the altar of the Tabernacle. Let's break it down:

"On the third day...": This indicates the day in the sequence of the offerings. The leaders of each tribe brought their offerings on consecutive days over a twelve-day period.
"Eliab the son of Helon...": This identifies the individual bringing the offering on the third day. Eliab is his personal name, and Helon is his father's name. This identifies him genealogically.
"...prince of the children of Zebulun": This clarifies Eliab's role and tribal affiliation. He was the leader (prince or chieftain) representing the tribe of Zebulun. This position would have held authority and responsibility within the tribe.

In essence, the verse simply states that on the third day of the altar dedication, Eliab, the son of Helon, representing and leading the tribe of Zebulun, brought his offering.

Significance in context:

The passage is important because:

It emphasizes the unity of the tribes: Each tribe participates equally in dedicating the Tabernacle.
It demonstrates the commitment of the leaders: They bring valuable offerings as a symbol of their dedication to God.
It details the specific offerings: While the verse itself doesn't specify the offering, the surrounding verses describe the silver platter, silver bowl, golden spoon, burnt offering, sin offering, and peace offering brought by each leader.
It highlights the meticulous record-keeping: The repetition and detail emphasize the importance of accurate records in the context of the Tabernacle and worship.

Therefore, the verse is a specific detail within a larger narrative that showcases the dedication and participation of each tribe in the establishment of the Tabernacle and its worship.