Well, with tiers people mean where it ranks approximately. Tier 1 are the decks that have a very good shot at all times to win tournaments. In the DP-Stormfront meta, decks like Kingdra, Dusknoir, Regigigas, those would be counted.
Moving down a bit you find Tier 1.5, which basically consists of decks that can still definitely be scary, but aren't played in as huge numbers. Again in the DP-Stormfront meta, one could rank AMU, Magnezone, GG and a few others.
Moving down further is Tier 2. Tier 2 decks are always interesting and can snag a victory or two, but generally aren't as feared as the tiers above it. These decks aren't the main worries, and tend to take advantage of a specific meta in order to pick their tournaments. Decks like Infernape, Gliscor, Raichu, those would be Tier 2-ish in the DP-SF meta.
We can't be sure about the DP-Platinum meta just yet - we can only predict which tier will host which decks. Tiers slowly settle over the course of the meta, rather then be insta-definable. For example, Regigigas was thought to be a fun rogue deck, not worthy of major tourney play, but proved to be among the strongest contestants. Raichu, on the other hand, recieved a lot of Tier 1 hype.
