An AECACHE file is an cache file created by After Effects, an application used to create visual effects and motion graphics for video. It stores recently used information, such as saved processed and rendered frames, so that it can be quickly accessed at a later time to improve the performance of the application.

Caching is a process used by After Effects where the application saves information from changes made by the user that can be reused to run more efficiently and improve performance. For example, when you render an image for the first time, the application will store information about that render so when you render again it will be quicker. Sometimes cache files, which include AECACHE files, grow to large sizes over time and burden your computer. And since After Effects does disk caching in the background you may not realize this problem. You can clean your disk cache however, by selecting Preferences → Media & Disk Cache and then clicking Empty Disk Cache. If this operation does not delete your AECACHE files, the application may still be using the cache files. To manually clean your cache you will need to navigate to your cache folder and delete the files.