I'm not a car guy, but I am the son of one and have plenty of gear head friends. DPF is Diesel Particulate Filter. It basically catches all the soot and then, when it's full enough, and the DPF is hot enough, there is a fuel injector that injects fuel and burns it all out. That's called a regen. When see white smoke, that's probably what you are seeing. If the DPF gets clogged, the engine won't start as it can't exaughst. So being able to regen is important. To get up to temperature to regen, the diesel needs to be run hard for about 10-15 minutes. Which it will get on a decent speed trip. DPFs came in to make diesels less dirty. Though you can argue it's moving the problem as much as reducing it. It's illegal but common, to bypass DPFs when it starts being an issue.

I know all this because it was the DPF that killed my last diesel.

Very widely held view that diesels were more reliable without DPFs. That they aren't good for the mileage they were pre-DPFs. Of course they were also even dirtier.