Resolution is the amount of information in a single frame of video, given as the number of logical pixels in each dimension. For example, a resolution of 1920 by 1080 works out to 1080 stacked horizontal lines, each of which is one logical pixel high and 1920 logical pixels wide. This resolution is frequently abbreviated 1080p because technically the width can vary. The dimensions 1080 by 1920 produce an aspect ratio of 16:9, which is the ratio of movie screens and modern television sets. By the way this is the resolution defined as full HD.
YouTube recommends the following resolutions for video uploads, all in the 16:9 aspect ratio. There's nothing specific to YouTube about this list. It's just a list of common 16:9 video resolutions.
|2160p||3840 x 2160|
|1440p||2560 x 1440|
|1080p||1920 x 1080|
|720p||1280 x 720|
|480p||854 x 480|
|360p||640 x 360|
|240p||426 x 240|
Which one should you use? That depends on your application. For simple embedding you may decide to chose only a single resolution. If you're preparing files for DASH or HLS, you may chose one, several, or all. Fortunately, this is one of the simplest transformations you'll make with FFmpeg.
If you don't have FFmpeg installed read Media application basics to get it set up with Docker.
/media # ffmpeg -i glocken.mov -b:v 350k -b:a 64k -s 1280x720 glocken_3g_720p.mp4
/media # ffmpeg -i glocken.mov -b:v 350k -b:a 64k -s 1280x720 glocken_3g_720p.webm
The following files should now exist:
/media # ls -l
-rw-r--r-- 1 root root 12080306 Mar 7 12:16 glocken.mov
-rwx------ 1 root root 531117 Mar 7 13:42 glocken_3g.mp4
-rwx------ 1 root root 706119 Mar 7 13:46 glocken_3g.webm
-rwx------ 1 root root 539414 Mar 7 14:15 glocken_3g_720p.mp4
-rwx------ 1 root root 735930 Mar 7 14:19 glocken_3g_720p.webm
It's worth reiterating that you should start from the highest resolution and bitrate file you have available. If you're upgrading an older site, you'll want to find your original camera or other high resolution sources and convert from that rather than from older web files.
Now that your files are prepared, you can either add them to a web page as they are now or dive a bit deeper and continue to learn more command line options by reading the Media conversion page, and then close out the section with Media encryption.