Mainly, Primarily
adverb
主 means main so when you add に it becomes the adverb mainly.
おもに
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you: You mainly spend your time talking to your master Moe. But you mainly address him like, "Oh Moe The Great!" (おも). That's because he's above you. Imagine yourself saying "OH MOE THE GREAT!" as you walk into his room, bowing low.
不正アクセスは主にヨーロッパからでした。
Unauthorized access was mainly from Europe.
日本の人口は主にアジア人です。
Japan's population is primarily Asian.
休日は主に山か川にいます。
I mainly spend my holidays in the mountains or at the river.