Magnets, in DAZ Studio parlance, are tools which take an object, and alter it’s shape in some way by pushing and pulling it toward (or away from) a particular point. Just like a magnet, in other words.
They’re most commonly used to make clothes fit after you’ve morphed the figure in some way. I could show you how to use them – but I’m not.
Elton has already beaten me to it with his rather spiffy tutorial showing how to use Magnets in DAZ Studio, so if you want to know more, head over there and tell him Greywulf sent you :D