This tool is used to convert Web Media (webm) to Matroska Multimedia Container (mkv).

WebM is an audiovisual media file format. It is primarily intended to offer a royalty-free alternative to use in the HTML5 video and the HTML5 audio elements. It has a sister project WebP for images.

MKV is a free, open standard container format that can hold an unlimited number of video, audio, picture, or subtitle tracks in one file.