One of the exhibits at the Evolving English exhibition at the British Library is a (modern!) recording of what is believed to be the oldest existing English round, Sumer Is Icumen In, that probably dates back to the mid-13th century. The title means 'Summer has arrived' (or come in). You can listen to the song at the exhibition via headphones. By doing so you will get an idea how Middle English sounded. The song is said to have been written in the Wessex dialect, which was the standard literary language of Anglo-Saxon England.
A round is a song for a group of singers, where one starts, then another starts a line later, then the third starts a line after that etc, so that at any one time everyone is singing a different line. You may well have sung Frère Jacques as a round.