Did you mean (j)obs->(m)anager, or are you looking at something from DF Hack? Edit: Looks like you can also get there from Units.
The most important thing to realize is that the quantity defines the size of a single batch. It starts in CHECKING mode and goes to ACTIVE when all conditions are met. It will stay in ACTIVE and not return to CHECKING until it completes the batch, so don't use infinite or it will never check the conditions again. Setting the frequency to daily/monthly/yearly, etc. lets you decide to leave the order inactive for a while before checking again.
Generally, pressing (r)eagents from the (c)onditions screen gives you good presets for inputs, which you can then tweak to your liking. (p)roducts gives you presets for outputs, so you can limit overproduction. Each entry consists of an item, material, and traits. Item is a base item, such as 'bolts', 'bars', 'liquid', or just 'item' for any type. Material is very specific, such as 'orthoclase', 'jabberer bone', or 'steel'. Traits are the most useful, containing such entries as 'bone', 'soap', 'metal', 'unrotten', etc., and they can be combined with each other. Going back outside to the manager screen, pressing (d) leads to the details screen, which is where you set job details like the engraving on a statue, or the material the job will actually use after it's done checking conditions. The conditions ignore hauled items (read: more bin problems.)
An example where you might use all three is: "unused dyed hemp cloth".
"Cloth" is the item type. If left as "item", we'd also get threads.
"Hemp" defines material. We don't want rope reed, cave spider silk, etc.
"Unused" and "dyed" are traits. "Unused" defines that it hasn't been partially used in a hospital, which would make it invalid for clothesmaking. "Dyed" means we don't want un-dyed cloth.
You'd want to set the cloth type on the details screen to "hemp", or they'd attempt to fill the order with all kinds of cloth despite all the trouble of checking for the existence of hemp. Unfortunately, there's no job detail for dyed cloth yet, so we're stuck using stockpile links in this example regardless.
I have some more practical examples in this post. I use some shorthand for the inequalities (e.g., <= means "at most", = means "exactly".)