The (1) indicates this is the second time a file with this name was saved to the same location. You may want to check your original 720.mp4 file to see if they are identical or different parts of a series.
The filename is a generic name typically generated by web browsers or download managers when a file named "720.mp4" already exists in your downloads folder. The "720" likely refers to the 720p HD resolution ( pixels) of the video. 720(1).mp4
720p is considered "Standard HD." It strikes a balance between visual quality and smaller file sizes, making it ideal for mobile viewing or older hardware. The (1) indicates this is the second time