...I don't know what a bunch of things mean (like A)...
[On the side note it was a poor attempt to trick me to make 100 rutile from 150 copper bars. I don't know much about modding, but I know what that specific number means.]
It is 150 bar, because bars use sizes. The size of 1 bar is default 150, so a reaction using 1 bar will use a value of 150 for that particular string in the reaction.
Bars, liquids, and powders tend to have sizes of 150. Threads are 15000, and cloth is 10000. Things like boulders and logs don't have sizes, or rather their size is 1. This is basically designed so that hard coded things in the game (like thread used for suturing in hospitals, or soap bars used in cleaning) can use up portions of items without consuming the whole item.
The (A) you referred to is the
reagent identifier, it allows the reaction to reference back to that particular reagent if it needs to. For example, if you made a reaction that would take a piece of leather and turn it into a rope, using the reagent identifier allows you to tell the reaction to make the rope out of the same material as the leather. For the purpose of this example, the command to do that is [GET_MATERIAL_FROM_REAGENT:"reagent identifier":NONE]
To learn more about reactions, check out
http://dwarffortresswiki.org/index.php/DF2012:Reactions. With a little reading and experimenting, you will make reactions your bitch in no time!