Rain Boots
Rubber Boots, Riding Boots
noun
Long shoes aren't long in that the feet part are long (that wouldn't make sense). Instead, the top part is long to keep you dry when it's raining. Yes, these are rain boots.
In modern Japanese, 長靴 is most commonly used to mean rain boots or rubber boots. It can occasionally mean riding boots as well. But if you're referring to fashion boots or boots in general, you can just call them ブーツ.
The reading is a bit weird. Double kun'yomi, so think along the lines of 長い and 靴 getting added together. 靴 is the reading you learned with the kanji, however. It also gets rendaku'd.
コウイチはいつも雨の日はピカチュウの長靴をはいてくるよね。
Koichi, you always wear Pikachu rain boots on rainy days, don't you?
長靴って子供のものだけかと思ってたんだけど、最近じゃ大人用のお洒落な長靴も売ってるんだね。
I always thought rain boots are for kids, but these days, there are fashionable rain boots for adults too, aren't there?
病院のベッドに横になりながら、彼は明日妻にいい乗馬用の長靴を買ってやろうと思った。
While laying in his hospital bed that night, he decided he would get a nice pair of riding boots for his wife next day.