What it is
A cap on the number of ads the account can show
Limited ad serving means Google has put a cap on the number of ads your account can show. The site stays online and the account stays accessible, but fewer Google ads appear, and revenue drops with them.
AdSense describes two reasons. The first is an account under assessment: Google is still getting to know your traffic. The second is concern about invalid traffic, that is, clicks and impressions that can artificially inflate the advertiser's cost or the publisher's earnings.
In Ad Manager, the limit applies only to Google demand and third-party bids. Directly sold campaigns, house ads and waterfall third-party sources are not affected. That is why, on a site with Ad Manager, the drop may show up in only part of the report.