The Martian "atmosphere" is essentially a vacuum to humans. No one will ever be able to walk on Mars without a full space suit.
Mars might be the most off-Earth "hospitable" place for humans in the solar system. And yet it's really really inhospitable.
Mars looks alluringly like parts of the desert southwest of North America, with sunny days and sometimes blue skies. But photos belie the extreme inhospitability of the planet, and the fact that the pressure on Mars is for most intents and purposes smilar to that of outer space.
Lack of atmospheric pressure (it's well less than 1% of Earth's on Mars's surface) is perhaps the most immediate and consequential harsh condition for human life. Exposing our bodies to this pressure (which is essentially like exposing them to the full vacuum of space) would have some immediately terrible consequences, including:
Simultaneous vomiting, defecation, and urination as our pressurized insides push bodily fluids outward.
Boiling away of bodily surface liquids like saliva and eye fluid (water can't exist as a liquid at this pressure, which is a problem for water-based beings).
Extreme swelling of our bodies as dissolved gasses come out of solution and water-based liquids evaporate as internal pressure drops (again no liquid water at this pressure).
Deoxygenation of our blood as oxygen comes out solution, potentially permanently damaging our lungs and in any case leading to unconsciosness within seconds and death within minutes.