To start of what means IOU it is an acronym for "I Owe You"
it's a token issued on the XRP Ledger. This can be done by everyone, however there is a small catch. There needs to be a trust line between the issuer and the receiver before making a transaction. Both trust each other that the other party has the backing of the funds.
How to deposit IOU funds:
You want to deposit Bitcoin, Ethereum, Bitcoin Cash, Ethereum Classic, Gold Gram Dash, REP into your XRP account on Ledger. You can do so using Gatehub. Bitstamp is also in the market but only for Bitcoin.
What you need to do is to create an account and send the funds over to the deposit address. If that is all done the exchange will give out the currency as an IOU to your XRP account.
These exchanges can also issue fiat IOU's like the dollar and euro to your account. You can also do that using these exchanges or the exchange function inside the app. If you then want to cash out you need to have a verified account, also known as Know Your Custumer (KYC).
Keep in mind that we support these platforms and exchanges. But are not in charge of the currency pairs they offer.