. In Cisco terminology, "w7" identifies a standalone OS (allowing local management via CLI or Web GUI), whereas "w8" would indicate a "Lightweight" image that requires a controller.
The file is a Cisco IOS software image specifically for Autonomous (standalone) operation of Cisco Aironet access points . This specific image is commonly used for the Cisco Aironet 3500 Series and 1260 Series access points. Image Breakdown ap3g1-k9w7-tar.153-3.jf15.tar
Here is a breakdown of the content and context for this file: Technical Details ap3g1-k9w7-tar.153-3.jf15.tar . In Cisco terminology