I know a little late but,
1.You do have to fill out the tax information, or your payments will continue to be put on hold.
2.Impressions is how many times the advertisements has been viewed. It's basically every page visit/view. No other restrictions like unquie visitors, it could be 5 of them, viewing at 200 times each if it showed 1000. or it could be 1000 people viewing once. but not normally. Impressions is just how many times the page has been viewed.
Normally it's done in impressions or by clicks.