Source of this rumour is unknown, which makes it somewhat unlikely, but there it is. According to someone somewhere, Galaxy S9 might be the first Samsung smartphone to feature modular design. What is a modular design? Putting it simply, it adds magnetic pins to the back of the device, allowing a variety of modifications (mods) to be installed onto the smartphone. Those mods may include adding a 360 camera to the smartphone, turning it into a console controller, a loudspeaker and much more.
Again, this is just a rumour. There is no substantial proof to Samsung even pondering the idea of making Galaxy S9 a modular phone, much less the company actually working on it. Make with it what you wish.