Means one in front of another, as opposed to side-by-side. It comes from a tandem harness with two horses pulling with one in front of the other. So technically no matter how many people you put on a bike, if they are in a straight line, they are still in tandem. As opposed to a bike where you sit side-by-side, also known as a sociable.
I had to check this one out as on Friday I’m doing a somewhat crazy cycle on a home-made 6-man bike (all sitting in tandem) from London to Amsterdam for a great cause - KEEN London who provide one-to-one sports for children with special needs. Any donation you can make will make a huge difference to a small organisation. With just one day left, please donate here if you can