Adverbial use emphasizing a large difference in degree or quality. Often used with comparatives or expressions of superiority.
His ability is far superior to that of the other players.
This problem is much more difficult than the previous one.
a few levels; several grades
Noun use referring to a difference of several ranks, steps, or degrees. Less common than the adverbial use.
There is a difference of several levels between the two.
Compound of 数 (several) and 等 (grade, class). The exact historical derivation is uncertain, but the word literally suggests 'several grades' of difference.