Paradex is a decentralized exchange that's built on top of the 0x protocol. DEXs are great because you can trade cryptocurrency pairs without trusting an intermediary (think Mt. Gox for why this is needed). This move shows that Coinbase is serious about building a decentralized trading platform (unless this is just an acquihire which would be very disappointing). Really exciting news!
If there's no intermediary, what exactly have Coinbase bought?
There is still an intermediary, they just never touch your funds.