What is the prefix ity mean?

The suffix "-ity" is not a prefix, but a suffix. It is used to form nouns that indicate:

* State or condition: For example, "clarity" (state of being clear), "opacity" (state of being opaque), "sanity" (state of being sane)

* Quality or characteristic: For example, "celebrity" (quality of being famous), "reality" (quality of being real), "velocity" (quality of being fast)

* Degree or extent: For example, "intensity" (degree of intensity), "quantity" (amount), "rarity" (degree of being rare)

Essentially, "-ity" often signifies the "state of being" or "quality of" the word it is attached to.