I think it’s understandable to be confused by it at first, but I think it’s a very cool mechanic that makes perfect sense. The easiest way to get it is to think of it in three parts.

1. How many dice do you get to roll?
2. What does each dice need to roll to be a success?
3. How many successes do you need to do the thing?

So you are using a skill in order to attempt doing a thing. That skill is dominated by an innate trait. (Forgery skill is dominated by Dexterity for example). So with these two things known, you can then answer the first three questions.

1. The trait score (Dexterity) tells you how many dice to roll. (DEX of 5 = 5 dice rolled)
2. The skill score (Forgery) tells you what you need to roll. Forgery score of 7 means each dice that is a 7 or lower is a success.
3. The difficulty rating of the thing you want to do tells you how many successes you need to do it. An easy task with a DR of 2 means you only need to roll 2 dice at your skill level or below.

So to recap. You want to forge a document that’s pretty easy to forge (a DR of 2 out of 10). So you roll 5 dice and 2 of those need to be a 7 or under (using the example scores above).