The dice level refers to the size of the dice to be rolled within an entry. Dice entries are provided in the format XdY, where X is the dice number and Y is the number of sides of the dice to be rolled.
Unlike dice number, a dice entry does not have a specific dice level that corresponds precisely to the number of sides. Rather, an effect that increases or decreases the dice level would increase or decrease the number of sides of the dice to roll, to a minimum of d2 and maximum of d12 (a d20 cannot have its dice level increased or decreased unless otherwise specified). The following list provides the complete scale of dice levels:
Therefore, increasing the dice level of 1d6 by 1 would result in a roll of 1d8, while decreasing the dice level of 1d6 by 1 would instead result in a roll of 1d4.
Specifically, you can multiply the dice level adjustment by 2, then add or subtract as needed to the base number of sides to get the resulting roll (to a minimum of d2 and maximum of d12). So, increasing the dice level by 2 would result in rolling dice with 4 more sides.
It is not typically allowed to reduce a dice below d2. However, Piercing and some other effects may permit a reduction beyond d2. In these cases, any entry reduced to less than a d2 is removed. It is never permitted to have d1, d0, or negative-size dice entries. If an opposed roll entry is removed as a result of dice level reductions, it automatically fails rather than automatically taking effect.