IPA: /ðɛrˈʌndər/
KK: /ˈðɛrˌʌndər/
In a position or situation that is below or beneath something else.
The documents were stored thereunder for easy access.
Thereunder → It is formed from "there" (from Old English "þær", meaning at that place) and "under" (from Old English "under", meaning beneath or below). The word "thereunder" means in or under that place.
Think of being 'at that place' ('there') and 'beneath' ('under') — that's what 'thereunder' means.