Glad it wasn't just me! I had to google it.
It's basically where you get a load of fans to hold up say, pieces of card or something which when all done together either spells out a message or picture etc. You'll have seen it loads of times, as I have, but like you, I'd no idea it was called a Tifo. Example below from when Spurs did it for Harry Kane although there's much more impressive ones from abroad if you google it. No doubt anything that Rovers do will be equally underwhelming if the 'Rovers' in white bin bags in the Darwen End upper are anything to go by!.