Translated, it says something like:
“Your app is targeting API 25, and should instead be targeted 26 to ensure optimal security and effeciency. From august 2018, all new apps must be API 26
From November 2018 all app updates must be API 26”
As I understand it - do nothing for now (=ignore), and make sure to upgarde to SDK 28 before releasing an update/new app in november/august, respectively