Not sure which part of that confuses you, so I’ll go farther into detail for the whole thing:
(Read whichever section was confusing you before)
You cannot use Derpibooru itself to make webm’s
Derpibooru is not an art program, nor does it have a way to change things into webm format. If you want to make a webm and whatever program you’re using doesn’t support webm you can make the video in a different format (MP4 is a common one) and then use any converter tool (Like the one
Smiling Pony sent.) and the converter tool will switch the video into webm format, making it compatible with Derpibooru.
**If you have an mp4 or other format video you wish to upload but don’t know how to switch it to webm, you can use any video converter tool to change it into webm format. **
If you already have the video you want to upload but it’s not in webm format just put it into any Webm converter
and it’ll change the video into a webm video, which you can then upload if that’s what you’re trying to do. These converters change the format in a way so the video still works without affecting the video itself much, since Derpibooru only supports webm’s you would need to do this before uploading.
If you still have a question please respond about it in detail, rather than “Huh?” so I can directly answer it instead of guessing what might have been confusing.