IPA: /ˈhɪərbaɪ/
KK: /ˈhɪrˌbaɪ/
This word is used to indicate that something is being done as a result of a specific action or document.
I hereby declare the meeting open.
Hereby is formed from "here" (from Old English "hēr", meaning in this place) and "by" (from Old English "bi", meaning near or beside). The word "hereby" means by means of this place or in this context.
Think of 'here' as indicating a specific place and 'by' as meaning near or in relation to it — that's how 'hereby' conveys the idea of something being done in this context.