Seeing the child in danger, he didn't think twice before jumping into the river to help.

didn't 是 do not 的缩写形式，表示否定。 think 的意思是"思考、考虑"。 twice 的意思是"两次"，这里 think twice 字面意思是"思考两次"。 didn't think twice 是一个固定短语，表示"毫不犹豫、不假思索"，形容立即采取行动而没有过多考虑。