To not take someone or something seriously; to underestimate or take lightly.
相手を甘く見てはいけない。
You must not underestimate your opponent.
彼は試験を甘く見ていたので、結果が散々だった。
He took the exam lightly, so his results were terrible.
新しいウイルスを甘く見ると大変なことになる。
It will be serious if you underestimate the new virus.
仕事を甘く見ず、しっかり準備しよう。
Don't take the job lightly; let's prepare properly.