The real short version: Basically you film an ad that usually runs at the start of your video, the client will usually provide "talking topics or a script of sorts" as well as video and annotation to included in your video, It is encouraged to try creatively incorporate it as part of your video so it doesn't seem like an Ad. Pros and Cons:
Pro is the CPM on these are really high, I think one just went for $25 which means for every 1000 views you get you get $25.
Cons: The ad champaign doesn't last forever and has a budget as well, so for example after 16 days or all the money runs out, the CPM goes away and your essentially not making any money off the ad at the start of your video. You can have normal monetization but as mentioned before you are kinda running two ads in a sense and your audience may not react well.
I find It's really up to the individual though, if you have a video that performs well you can make some serious money, so its worth considering