Oxygen

is essential for humans to linger and call Mars their home. It can be produced using the sophisticated MOXIE buildings and later on, naturally generated from crops.

Background
Oxygen is the most short term life support resource. If colonists are left without oxygen they will die extremely quickly. And while Mars's atmosphere has little to no Oxygen, what it does provide is an abundance of Carbon Dioxide, which our brilliant MOXIE technology can convert to the much needed Oxygen.