A closer look at TrueType hinting
Tim Ahrens, writing for Typekit:
TrueType hinting — sometimes more correctly called instructions — bends the contour of the letter into a slightly different shape before it is rasterized.
Finding a good hinting strategy can be tricky at times. Some more complex letters can require special attention, and design-related questions arise: do we want to keep the black strokes consistent, or should we preserve the white inner space? In extra bold fonts such as the “s” above, one might instead choose to preserve the relationship between the black and white parts of the letter. The interplay of black and white is at the very heart of type design (and, likewise, of reading).