After this date, Skype for Business Online (SfBO) will no longer be available or accessible.
If you are using Skype for Business Online, you need to plan now your migration to Microsoft Teams.
Between now and this retirement date, Skype for Business Online will not receive any service updates – meaning no new features or improvements, off course any security issue (if any) will be fixed.
Until the retirement date, you can continue to use Skype for Business and add new users to the service. In the interim, starting September 1st 2019 new Office 365 customer will be directly onboarded to Teams.
You can start looking at documentation to help you plan and migrate to Teams here https://docs.microsoft.com/en-gb/MicrosoftTeams/upgrade-start-here while you can also start sharing with your end-users this self-service training page to start learning about Teams https://docs.microsoft.com/en-us/microsoftteams/instructor-led-training-teams-landing-page
IMPORTANT this retirement does not concern Skype for Business on-premises and/or Skype consumer.