"Raiding" is the term given to the act of numerous players coming together in order to kill (or, “down”) bosses.  A typical raid tier will usually include between ten and seventeen bosses.  When one refers to a “raid tier,” s/he is referring to a specific level of boss encounters.  For example, the game will usually have an introductory raid tier, a middle tier about 4-6 months after this, and then a final tier after another 4-6 months.  The goal of most raiders is to clear all the bosses in the introductory tier before the middle tier is released, and, likewise, to clear all the middle-tier bosses before the final tier is released.  This cycle of introductory, middle, and final raid tiers repeats approximately every 2 years when a new major update is made to the game.

 

The concept of raid tier progression gets slightly more complicated when considering the different difficulty levels for each tier.  At the moment, there are 4 levels per tier: (in order of increasing difficulty) Raid Finder, Normal, Heroic, and Mythic.  As more tiers are introduced, raid teams may progress across different difficulties.  Sometimes, clearing Heroic difficulty in the introductory raid tier might allow a raid team to start with Normal or Heroic-difficulty in the middle raid tier.

 

The diagram below is a simplified way of visualizing how a group might progress between raid tiers.