hauptsächlich
mainly
Expresses that something is the most important or predominant part of something; above all, primarily.
Ich ernähre mich hauptsächlich von Gemüse und Obst.
I mainly eat vegetables and fruit.

mainly
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mainly
Expresses that something is the most important or predominant part of something; above all, primarily.
Ich ernähre mich hauptsächlich von Gemüse und Obst.
I mainly eat vegetables and fruit.

The word is composed of 'Haupt' (main, head) and 'Sache' (thing, matter). 'Hauptsächlich' therefore literally means 'as far as the main matter is concerned' and emphasizes what is most important.
Die Hauptsache ist, dass alle gesund sind.
The main thing is that everyone is healthy.
As an adverb, 'hauptsächlich' can stand flexibly in the sentence. If it stands in position 1, the verb follows in second position. If it stands inside the sentence, it usually emphasizes what comes directly after it.
Hauptsächlich trinke ich morgens Kaffee.
Mainly, I drink coffee in the morning.
'Hauptsächlich' can also be used as an adjective before a noun. It then means 'main' or 'primary' and is declined like a normal adjective.
Der hauptsächliche Grund für die Verspätung war der Stau.
The main reason for the delay was the traffic jam.
all
'vor allem' with 'all' and 'hauptsächlich' can both show what is especially important. 'vor allem' highlights something and often sounds a bit more direct: you are saying what matters especially. 'hauptsächlich' tends to describe the largest or most important part of something. It fits well when you are explaining what something mostly consists of or what someone is most occupied with. The two are similar, but 'vor allem' emphasizes more strongly, while 'hauptsächlich' classifies things more factually.

Im Zug lese ich hauptsächlich Nachrichten und kurze Texte.
On the train, I mainly read news and short texts.
Zu Hause lernen wir vor allem für die Prüfung am Freitag.
At home, we are studying mainly for the exam on Friday.