Lumber
Timber, Wood
noun
Lumber trees are what you use to make lumber and timber.
材木 is typically for lumber or timber that's been processed — the stuff that's cut into planks and ready to use in your next DIY project, for example.
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.
なんで材木にクイックルワイパーをかけてるの？
Why are you mopping that wood with a Quickle wiper?
明日の朝一番に材木屋さんに電話してみるよ。
I'll try calling the lumberyard first thing tomorrow morning.
このトラックで、ここから材木店まで材木を運ぶんです。
With this truck, we transport lumber from here to the lumberyard.