A colloquial contraction of 'kore wa', used as an interjection to express surprise, draw attention, or exclaim.
こりゃまずいね、すぐ対処しないと。
This is bad — we need to deal with it right away.
こりゃ君、よくやったよ。
Well, you did a good job.
こりゃ大変だ。早く助けを呼ぼう。
This is serious. Let's call for help quickly.
こりゃ高いなあ、もっと安くならないかな。
This is expensive — I wonder if it can be cheaper.