Buffs are bonuses to your rolls, and hence very useful. There are two types of Buff: there is the +Buff and the dBuff. They work slightly differently.

+Buffs add a fixed number to your result after you roll the doubling die.

dBuffs add an extra die to your roll before you roll the doubling die.

For example, if you are rolling a D6 and you get (for whatever reason) a +Buff of 3, then you will roll D6 (remember, the captial D means it's a doubled roll) and you will add 3 to the result. If you roll a 4 on the d6 and a 2 on the DD, then your result will be 9 (4x2+3).

If you are rolling a D6 and you get a dBuff of 3, this means that you will roll both a d6 AND a d3, and multiply the result by the DD. Say you roll a 4 on the d6, a 2 on the d3 and a 2 on the DD, then your result will be (4+2)x2 = 12.

Clear as mud? Don't worry, it does make sense, really. It does get a little more complicated, but not until you are higher levels. When you reach certain levels, the Buff table changes. Buffs are given as RCs, not direct rolls. So above, it should have been +Buff(RC3). The reason I didn't do it like that is that it doesn't matter at low levels. However. Suffice for the moment to say that as the characters increase in power, Buffs increase in power as well. A Buff(RC3) will mean one thing to a Level-0 character, but something more to a Level-4 character. Don't worry about that - there's (all together now) a table on the character sheet.

Masteries and Meta can give you Buffs, as can certain equipment.

Don't worry, you'll get used to it.