Depth, Profundity
noun
This is the noun version of the kanji deep, so it means depth.
ふかさ
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You already learned this reading when you learned 深い so you should be able to read this one too!
このプールの深さを教えて下さい。
Could you tell me the depth of this pool?
意外と深さがあってびっくりしたよ。
I was surprised by how deep it was.
百メートルの深さまでハシゴで下りました。
I climbed down a ladder to a depth of 100 meters.