Okay, Colin, I'll have a go.
The early collectors went round recording what they decided to call folk songs, those songs that had been passed down in oral tradition, some dating back to the 16th century right through to the early 19th century. Most were printed on broadsides and had a certain style, narrative, simple structure, simple tunes, few characters, often lots of dialogue, and other characteristics, covered certain subjects. To emulate this in your own song writing would then be 'in the traditional style'. This is very loosely defined by the way.

Some writers write about heritage subjects, some about topical events, some use traditional structures and tunes.

Hope this helps.