Considerably is from considerable plus -ly. The suffix -ly recasts considerable as an adverb: the lexical idea remains recognizable, but the word now describes manner, degree, or circumstance rather than naming the quality by itself. In use as an adverb, considerably denotes by a large amount or to a great degree. Typical contexts include “The situation has improved considerably since last year”. The meaning stays close to the form: it applies the etymological material to by a large amount or to a great degree, keeping the visible parts legible in ordinary use.