Space radiation is a hazard to humans in orbit outside the Earth, and it would pose an even bigger threat to astronauts on Mars. The "Red Planet" is closer to the sun and gets much more radiation than Earth, which could theoretically be deadly for Mars astronauts. Still, experts aren't certain as to how this radiation would affect humans.
NASA and other spaceflight institutions are committed to finding safe ways for humans to land on Mars. There are some practical approaches, including timing a Mars-bound spaceflight to coincide with a relatively safer point during the sun's natural cycle when the sun's radiation is less intense within the solar system.
Even if astronauts on Mars were not well-protected from space radiation, they would not necessarily die from the exposure. Most likely, they would be highly susceptible to cancer that could be treated or even cured. Astronauts on Mars would still need to take extreme precautions, and the spaceflight organizations still need to do extensive research to make sure humans would be safe for an extended stay on Mars.