Important
Valuable, Serious Matter
noun, な adjective
A big action means something big is going down. This is no ordinary action. It is an action that is important.
This is a jukugo word, which usually means on'yomi readings from the kanji. Since 大 has two on'yomi readings, here's a mnemonic to help you remember which one to use:
Nothing in your life is as important or valuable to you as dyes (だい). You really just love dyes more than anything else. Dyes, dyes, dyes!
きちんと「ありがとう」と言うことは大事です。
It is important to say "thank you" properly.
大事なノートを失ってしまった。
I lost an important notebook.
明日は大事な大会なので、早くベッドに行きます。
Tomorrow is an important tournament, so I will go to bed soon.