MKVToolnix is a free and open-source software that can be used to create, inspect, and edit Matroska multimedia files. It allows users to merge multiple video, audio, and subtitle tracks into a single file, and split them into separate files. The software also provides advanced features such as adding or removing specific tracks, changing the aspect ratio, and adjusting the compression settings. Additionally, it supports various input and output formats, including AVI, MP4, MPEG, and more.