Basically, it affects the smoothness of you lines. 0 disables the feature, 1-15 is more for fast drawing as the lines are generally less smooth (i.e. sketching), and s-1-s-7 makes your lines more smooth, as the cursor moves slower when drawing. It's only relevant if you have a tablet, though.
No, that is the name I gave to the brush I created. It's actually an airbrush, but it gets confusing if I don't give each brush a specific name (since I have so many ;A; ). I called it Line because that's what it was intended for (I'm so imaginative!! :'D). The Lineart tool, on the other hand, is generally more suited to mouse users than tablet (although tablet users can use it too), as it allows you to play with the pressure and whatnot in a way that mouse users can't when using the layer tool.