Numbers 6:5
All the days of the vow of his separation there shall no razor come upon his head: until the days be fulfilled, in the which he separateth himself unto the LORD, he shall be holy, and shall let the locks of the hair of his head grow. All the days of the vow of his separation there shall no razor come upon his head: until the days be fulfilled, in the which he separateth himself unto the LORD, he shall be holy, and shall let the locks of the hair of his head grow.
The vow is a separation to the Lord for an appointed period, during which no razor is to touch the head, the locks are left to grow, and the person remains holy until the days are fulfilled. The uncut hair is the outward token of that consecration to God, marking a sacred calling and loyalty. Strength does not reside in the length of the hair itself, but in the consecration and blessing it signifies.
Samson disclosed the token when he made known that no razor had come upon his head. By permitting his hair to be cut after betraying his calling, he surrendered the blessing signified by it, and the Lord departed from him. John was dedicated to God as a Nazirite from birth and took the vow as his own lifelong consecration, letting his hair grow through his whole period of separation to the Lord.
Four believers in Jerusalem were approaching the close of such vows and still lacked the required purification ceremonies. Paul met their expenses and entered with them into the temple observances for seven days.