IPA: /ðeəˈrɑːftər/
KK: /ðɛrˈæftər/
From a certain point in time onward; after that moment.
She graduated from college and thereafter started her own business.
Thereafter is formed from "there" (from Old English "þær", meaning at that place) and "after" (from Old English "æfter", meaning following in time or place). The word "thereafter" means at that place or time following a specified event or situation.
Think of a specific place or time ('there') that comes 'after' something else — that's what thereafter means.