Grab ffmpeg from https://www.ffmpeg.org/download.html
It's a command line tool which means you will have to type things with your keyboard instead of clicking on buttons.
The most trivial operation would be converting gifs:
ffmpeg -i your_gif.gif -c:v libvpx -crf 12 -b:v 500K output.webm
-crfvalues can go from 4 to 63. Lower values mean better quality.
-b:vis the maximum allowed bitrate. Higher means better quality.