Add the files you want to join, put them in the order you want them played, and export the result as a single track. MP3, WAV, FLAC and M4A all work, and you can mix formats in one job — everything is decoded to the same internal format before it is joined, so a WAV and an MP3 sit together without a seam.
The merge happens in your browser using the Web Audio API. Your files are never uploaded, which is why there is no size limit, no queue and no watermark: nothing leaves your device, so there is nothing for us to charge for or to store.
One thing worth knowing: MP3 encoding adds a short silence at the start and end of every file. Joining MP3s can therefore leave small gaps you did not put there. If you need a seamless join — a DJ mix, a continuous recording — use WAV or FLAC sources and export once at the end.