Tiers of play groupings of levels that identify the overall power and influence of an adventurer or party. In the current edition of the game, the tiers are divided as: Levels 1 - 4, levels 5 - 10, levels 11 - 16, and levels 17 - 20. Unlike 4th edition, these tiers to not have names, but asre simply identified as "Fiert Tier," Seond Tier," etc.
The text of the Player's Handbook identifies the First Tier as "apprentice" and implies the Second Tier could be understood as "Master" ("characters come into their own"). Building from these, Third Tier is "Grand Master," and Fourth Tier is "Paragon."
5th level as the beginning of Master tier supports my theory (introduced in this post) that "Name Level" since 3rd edition is 5th level.