You must be referring to the Glyph Transform dialogue.
Have you read the tutorial on Using Glyph Transformations?
The Bold Effect increases the weight to horizontal and vertical strokes. The Thin effect reduces the weight of the strokes. The Cap Height will also increase/decrease as a side effect, so scaling is also required to maintain consistent font metrics across font styles.
It would be better if bolding and thinning were proportional to stroke weight