This refers to the vertical resolution: 1280x720 pixels. In the world of compression, 720p is the "sweet spot" for mobile devices, older HDTVs, and users with limited bandwidth. It offers a significant upgrade over 480p DVD quality while maintaining a file size roughly 70% smaller than 1080p.
For the data enthusiasts, here is what a standard Proper release of this spec usually contains: Justice League 2017 HC 720p HDRip 850 MB
This filename matches patterns used for unauthorized copies; possession/distribution may violate copyright law in many jurisdictions. Obtain media through legal channels when possible. This refers to the vertical resolution: 1280x720 pixels