IPA: /hɪˈrɪn/
KK: /hɪˈrin/
In or into this document or statement.
The terms of the agreement are outlined herein.
Herein is formed from "here" (from Old English "hēr", meaning in this place) and "in" (from Old English "in", meaning within). The word "herein" means in this place or within this document or context.
Think of 'here' meaning in this place and 'in' meaning within — together they help you remember that 'herein' refers to something contained within this place or context.