It'll be the type of thing that knights and damsels danced about to, or sang at each other when they were happy, or dying of love, or something...
...oh, all right, I'll look it up if you want to know exactly...
The good news is that virelays (or virelais, if you like) went out of date around the end of the 1400s and they were only ever really popular in French, so ignorance of them isn't likely to have caused anyone born in the last five hundred years much damage.
They started off being sung, did virelays, but towards the end they were written purely as poetry. They usually had three verses with a chorus sung first and last and in between (though you call the choruses and verses refrains and stanzas if you're feeling fussy).
Anyway, you sing your chorus, then you sing a line of verse to a different tune, then another line of verse to the same tune as the line of verse you've just sung, then you sing the last line of verse to the tune of the chorus, and then you sing the chorus again.
If you're being particularly clever (and why not) then you'll do the whole thing, all three verses and choruses, using only a couple of different rhymes.
(Good grief, that sounds tricky, I'm suddenly rather glad the thing went out of fashion in the late 1400s.)
As I said, virelays are almost all in French, so it's surprising that the technical term for the first two lines of the verses is stollen, and for the last line the abgesang.
But, hey, I suppose the French were too busy singing to stop to analyse why they were enjoying themselves so much.
Word To Use Today: virelay. The Old French form is virelai, which was a meaningless word used in choruses and is probably something to do with our word lay meaning a ballad. The German stollen means gallery as well as fruit cake; and abgesang is more or less the same thing as our swansong.