First, the direct answer: because air pressure is so low and gravity so weak on Mars, it’s a challenge but doable. We need to remove toxins from the air and increase the percent of oxygen. While Earth has a solid 21% oxygen, on Mars we’ll likely need a higher percent of oxygen due to the thinner air. So, the minimum needed is likely more than 21%, perhaps around 35%. To add atmosphere to Mars, there are two ways: we can add it externally or release it from Mars itself. The easiest way might be with plants that convert external Sun energy.
On Earth, we have 21% oxygen and 78% nitrogen, with a psi ranging from 14.7 at sea level to 7.93 at La Rinconada. People can acclimatize to these extremes well. On Mars, the air pressure on the ground is only 0.085 psi, which is 173 times thinner, and 95% carbon dioxide, which is toxic to us. The oxygen level is only 0.13%. The first step is to unlock the oxygen from the CO2, which both removes toxic carbon dioxide and increases oxygen. However, the biggest problem is the atmospheric pressure.
The gravity on Mars is only 38% that of Earth, meaning gases escape more easily. This means we’ll need to add an ozone layer and a magnetic field.
When we add atmosphere, we have to strive for above 21% oxygen with carbon dioxide levels below 1%. Over time, the Martian air might end up being something like 33% oxygen, 50% nitrogen, and 17% argon, giving it a slightly metallic scent. It might become common to say things like, “You like the smell of Earth, but I actually prefer Martian air.”