Physical Shortage, Insufficient, Not Sufficient, Not Enough
Noun, する verb, and な adjective
ふそく
おつりが不足しています。
I don’t have enough change.
しきん不足で中央せいふのきかくしたプロジェクトが中止になった。
Due to the lack of funds, the project that the central government planned was suspended.
経験が不足しているうえにいつも寝不足なので、あなたは首です。
You are fired because you lack experience and are always sleep deprived.
You have something that's not sufficient. When something isn't sufficient, you have a shortage. Keep in mind that this is a physical shortage, not a shortage of mental ability or something like that. To remember this, just think about how 足 also means "foot." What is a foot? Something physical. That's why this is a physical shortage.
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.