Apparently they are a decentralized ethereum asset exchange, which I have had no experience with.

But if any site tries to get you to give them your private keys, for whatever reason, even if it's for signing a smart contract, it's probably best to avoid them or at least try to find the best alternative that has a longer track record.

For example, Etherdelta seems to be widely used and probably the most popular decentralized exchange without much complaints and concern about security.

Even if you were to trade on Idex, make sure you use a disposable address so that you don't reveal the private key of the wallet that you usually send and receive funds on.