ok i'll try to explain it a little better.

take this compound for an example:

aluminum plate (R6) = aluminum ore (R3) + granite (R6)

since aluminum ore is the lowest rank (R3), the maximum ranks the result of this compound can increase with primary alchemy (without alchemy books) is +4. so you have the possibly of producing a R7 aluminum based item (hard aluminum plate). you also have the possibility of producing any aluminum based item between R1 and R7.

in this case the result is aluminum plate (R6) which is +3 ranks from the lowest item in the components...hence the compound says (Primary +3)

basically,

+4 is the hardest to obtain so getting a hard aluminum plate will be very hard.
+3 is a little easier since it is only 3 ranks higher then the lowest component
+2 and +1 and 0 or any negative numbers are the easiest to produce because the result does not have to rise as many ranks

so...higher + numbers mean the compound is harder to produce, lower numbers or negative numbers are easier to make.

higher alchemy skill will improve the chances of getting a higher rank item as a result.