gilded quail
Cosmic radiation and solar radiation. Mars does not have a magnetic field, ozone layer, or atmosphere to protect the surface from this kind of radiation. Cosmic rays are high energy particles (x-rays or gamma rays) and solar radiation is pretty much everything on the electromagnetic spectrum.
To survive you would need a suit that protects you from radiation (and of course everything else about mars trying to kill you), as well as a habitat that can do the same.
Being underground would be helpful, since most of that radiation can't penetrate too far into the surface.