They are substitution or pre-composed variant that are alternate glyphs and may not be available on all fonts; you will see them only when the font was created to incorporate them in a specific way.
Example from Myriad Pro Bold from the Typekit:
Top example is without using the "Fraction" substitution, bottom example is using it.
You can't really enable them if the font doesn't have these substitutions included already in their set. They will be accessible only for certain fonts. You'll find them on Open-Type fonts.
If they are available, you can enable or disable them by clicking on the icon.
There's also fonts that include these substitutions but they might be part of another font family named differently and won't be available on this small Character menu. They're not as convenient to use as the option above. For example, the Helvetica Fractions.