If you're talking about brew systems, it's the number of stands (tiers) for kettles to sit on. Usually consists of one tier with a gas burner (for boiling) and another to collect the finished wort using gravity to transfer. Most folks, from what I know, use the 3 tier to sparge on the highest tier first, then transfer to boil, and so on.
If you mean the
3-tier distribution system, that's a whole other can o' worms.