An exclamation of surprise or dismay meaning “what on earth is this?” Used when encountering something strange, unexpected, or hard to believe.
何だこれは！この部屋、誰がこんなに散らかしたんだ？
What on earth is this! Who messed up this room like this?
なんだこれは……味が変だ。いつもと違うぞ。
What is this… the taste is odd. It's different from usual.
漫画で怪物が突然現れて、主人公が『なんだこれは』と叫んだ。
In the comic a monster suddenly appeared and the protagonist yelled, 'What is this?'