Pivot reports contain a lot of information. Refer to this guide to make sense of the data.
The "Tail Aggregate" is compiled of all sites that have bid on 200 impressions or less. Once these sites go above 200 impressions, they break out into their own line item. This is done because there can be thousands of sites that make up the tail aggregate. To give each of these sites its own line item would make optimizing very difficult. Oftentimes, in the first few days of a campaign, only the tail aggregate will populate because no individual website has received 200 bids. If this is the case, you can reach out to the Growth Account team in the Support Center at firstname.lastname@example.org to request a tail aggregate report.
"Site-Not-Provided" is a phrase used to name sites when the publisher does not want their domain name shown in RTB reports. We frequently see "Site-Not-Provided" shown when higher-profile websites do not want their domain shown on the RTB reports, so brands may continue to buy ad placements directly from them.
Numbers in your pivot report, listed under Sites, are often the ID numbers for apps. Run a quick Google search of the number, and you'll usually be able to identify the application.