Fierce
Intense
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 "Intense _____," or something along those lines. If you see this, you know that something is being done really fiercely or intensely.
猛 is used in compounds to emphasize fierceness or intensity, like 猛暑 (intense heat) or 猛勉強 (intense studying). It can also be used in expressions like 猛アプローチ (relentlessly pursuing someone romantically — a fierce "approach"). In casual uses like this, the strong tone is often intentionally goofy or comical.
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 fiercely 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.