Fierce
Energetic
prefix
The kanji and the word are exactly the same. That means they share meanings as well. You can put this on other words to say that it's a "Fierce ____" or "Energetic _____," or something along those lines. If you see this, you know that something is done fiercely or really energetically.
The reading is the same as what you learned with the kanji, probably because it's stuck onto things making it all jukugo and such.
彼は猛勉強して、大学に合格しました。
He studied energetically and passed the university entrance exam.
猛ふぶきで雪が三メートル程積もりました。
A fierce blizzard piled up about three meters of snow.
あの犬、急に猛ダッシュしてきたからびっくりしたよ！
That dog suddenly dashed at me fiercely, and I was shocked!
バイクが猛スピードで突っ込んできた。
The motorcycle came rushing forward at breakneck speed.