Choose one of the following:
- OK - Disables the preference Use Flash Player For Playing 3D and Multimedia Content. FlashPlayer is not used to play flash content. The PDF uses the native OS media players, like Windows Media Player on Windows or QuickTime on macOS, to play the audio or video content in the PDF.
- Ask me later - The preference Use Flash Player For Playing 3D and Multimedia Content is enabled, and FlashPlayer plays the flash content. The message is displayed again if the user opens a PDF file with flash content.
For more information please take a look at the help page https://helpx.adobe.com/acrobat/kb/flash-format-support-in-pdf.html
Hope it will help