Compared to Earth, the atmosphere on Mars barely exists. The average air pressure on thesurface of Mars is about 600 pascals (0.6 kilopascals). That sounds like a big number, but it's actually less than 1% the air density of Earth. So there's very little air pressure.
Furthermore, the composition of the atmosphere on Mars is 95% carbon dioxide, 3% nitrogen, 1.6% argon and then trace amounts of oxygen and water vapor. And carbon dioxide is absolutely something you can't survive on. You need more than trace amounts of oxygen, you need the same levels we have here on Earth to breathe.