They spell it out pretty clearly in the PDF.

> The three metrics you guys need to care about is Click Thru Rate (CTR), Average View Duration (AVD), and Average View Percentage (AVP).

> How to measure the success of content

> Like I said at the start of this the metrics you care about in regards to virality are CTR, AVD, and AVP. If you want to know if the contents of a video are good, just look at the AVD and AVP of a video after we upload it.