https://www.gamespot.com/articles/starfield-contraband-stolen-items-guide/1100-6517297/

Encumbrance
Speaking of encumbrance, it works a little differently in Starfield than in other Bethesda RPGs. If you’re carrying too much, you can still move, sprint, shoot, and jump. However, your oxygen levels will decrease more quickly as you move around. When you run out of oxygen, you accumulate CO2 which will eat away at your health. CO2 buildup can’t kill you, but it can hurt you.

When you’re overencumbered, you can’t fast travel.