It means like, a 20 sided dice roll, with a 2 point modifier. Say you roll a 4, meaning you missed, but with the modifier, it modifies your point by two so you get a 6, so you hit the target. Modifiers usually do not count towards criticals so, if the range for a critical is 18-20, you must get those numbers on the natural roll, and not with modifiers. So 16+2 and 17+2 won't crit, but 18->20 will. 

And now that we're on the subject, will enemies have difficulty checks as well?