Apple is sending out email notices to everyone who bought Aperture in the Mac App Store, reminding them that the end is nigh.
Apple announced their intention to discontinue both Aperture and iPhoto back during WWDC 2014. This new notice reminds people that that's still happening — likely following the full consumer release of Photos for Mac at some point this spring —and that they will be able to migrate their Aperture library to Photos for Mac once it's released.
You can migrate your Aperture library to Photos for OS X, including your photos, adjustments, albums, and keywords. After migrating, your Aperture library remains intact. However, Aperture and Photos do not share a unified library, so any changes made after the migration will not be shared between the apps.
Photos for Mac is expected to arrive for most consumers with the release of OS X 10.10.3. The app has already made an appearance in the recent developer releases of OS X, as well as the public beta release earlier this week. If you have any questions about Photos for Mac in relation to Aperture be sure to check out our guide to What you need to know about Photos for OS X.