A magnetic rower uses a magnet in its flywheel to create resistance. Like its air rower counterpart, a magnetic rower creates resistance you will counter when pulling the handlebar connected to the fan flywheel via a strap.
While air rowers create resistance by displacing the air around the flywheel, magnetic rowers use a magnet's eddy current to resist the flywheel's spinning, thus creating resistance.
So, a magnetic rower features a magnet in its flywheel for braking. You will vary the resistance using a dial where you can increase (or decrease) the braking force or resistance level.