Short Answer: +, -, positive, negative, neutral, etc. are talking about nature.

Long Answer: 20 of the 25 natures have effects on stats, raising one by 10% and lowering one by 10%. There are tables that show which natures boost and hinder which stats, but you can easily see what stats a nature effects by looking at the stats of the Pokemon. If the name of the stat is red, the stat is boosted by nature. If the name is blue, the stat is hindered by nature. So, + and positive refer to when you have a nature that boosts that particular stat, - and negative refer to when you have a nature that hinders that particular stat, and neutral refers to when the nature doesn't affect that stat.

Hope this helps.

Yes, exactly. By the way, as you mention the calculator, to define your nature, you need to check which stat is positive and which one is negative, like Eevee explained. Once you know which on is positive and which one is negative, you can click the left click the positive IV column and right click the negative IV column. This will adapt the stats to that specific nature, but make sure to first setup your nature and then to select your stats :)