OK, each tier/type of tank (vertical axis, such as a tier 5 heavy (i.e. KV, T1 Heavy)) is listed. For every colored bar on that row, those are the different "combat levels" (I like to call it "rank") of matchmaking you may be put into. Each column is a different rank, and indicates what tiers/types of tanks can be found in that rank.
It's a bit confusing, but the easiest way to read it is to find the row for the tank you want to investigate. Figure out the range of ranks you may play in, and then see what other tanks are included in each of those ranks. The one you're probably most interested in is the max rank.
To better illustrate, let's take the example of the KV, a tier 5 heavy tank. A tier 5 heavy tank can be in ranks 4, 5, 6, or 7. In a rank 4 match, you will go up against any Tier 3 or 4 tank, and any Tier 5 non-artillery tank. However, you could also get placed in a rank 7 match, which would include Tier 4 light/artillery and any tanks from tier 5 to tier 9. However, in that rank 7 match you won't also see a tier 3 tank.
If instead you are in a rank 6 match, you could see a tier 3 arty, any tier 4 vehicle except a heavy (are there tier 4 heavies???), any tier 5 tank, and any non-arty tier 6, 7, or 8 tank.
Edit: Note that the colors in the grid are just used to distinguish the type of tank but don't indicate anything else. The bottom section seems to be for tanks that have exceptions to the rule. For example, the Bison_1 is a tier 2 SPG, but instead of being limited to rank 2 and 3 matches it can be found in anything up to rank 5.
I also found it interesting that they didn't include Tier 9 mediums -- aren't they releasing those in this patch as well?
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