IPA: /səbˈstænʧəli/
KK: /səbˈstænʧə·li/
In a significant or considerable way; to a large degree.
The project was substantially completed ahead of schedule.
Substantially is formed from "substantia" (meaning substance or essence) and the suffix "-ally" (meaning in a manner of). The word refers to something that is done in a manner that relates to the essence or substance of a matter.
Think of 'substance' as the core or essence of something, and remember that 'substantially' means doing something in a way that relates to that core essence.