Google will introduce tougher rules for advertising in apps and games, with the goal of making advertising more tolerable. All full-screen ads that appear unexpectedly, such as when the user has chosen to do something else, are prohibited.
Games may not display advertisements at the start of a new level or at the beginning of a “content segment”. Developers are no longer allowed to display full-screen ads during app or game startup. Any full-screen ads that cannot be closed after 15 seconds will be banned.
However, this does not apply to advertisements that the user has voluntarily chosen to see, for example to unlock something in the app or in the game. The changes come into force on 30 September. Google has written columns about other policy changes for developers, several of which improve privacy for users.
The text can be read here for those who are curious.
Examples of prohibited advertising