Resolution describes the number of pixels in each video frame. A 1920 x 1080 clip has more than twice as many pixels as a 1280 x 720 clip. Lowering resolution can make a video easier to play or share, although final file size also depends on duration, frame rate, codec, and bitrate.
This changer loads a WebAssembly video engine only after you start. It uses Lanczos resampling, outputs H.264 video with YUV 4:2:0 pixels, and carries the first audio stream into AAC. Subtitles, chapters, extra tracks, attachments, and source metadata are not copied. The output is always MP4 for broad browser and device support.