Wood
Lumber
noun
Wood that is lumber is lumber wood.
Technically speaking, 木材 is used more for lumber or wood that hasn't been processed yet — less like planks, more like freshly cut lumber still lying on the forest floor. But it's not a hard rule or anything, so don't worry too much about the nuances.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
木材を使う
to utilize wood
木材を使用する
to utilize wood
木材を用いる
to utilize wood
明日、ホームセンターに木材を買いに行く予定です。
I'm planning to go to the home center to buy lumber tomorrow.
こないだ初めて、木材でDIYしたんだ。
The other day, I tried a DIY with lumber for the first time.
えーっ、マジックアルハンブラに木材使ったものがあるの、知らなかった。かわいい！
Oh, I didn't know there was something made of wood in Magic Alhambra! Cute!