MKVToolnix is a free and open-source software for creating, editing, and inspecting Matroska (MKV) files. It allows users to add, remove, and manipulate audio, video, and subtitle tracks within an MKV file. The software also supports various video and audio codecs, chapters, and attachments. Additionally, it provides advanced features such as merging multiple MKV files and creating new Matroska files from other media formats.