Question

What does the subscript indicate in the chemical formula `CO_2`?

Answer 1

Clearly, it illustrates the physical state of the carbon dioxide: solid, gas, or (very rarely!) liquid.

Explanation:

Dry ice (aka card ice) sublimes at `-78` `""^@C`, i.e. it undergoes the transition from solid to gas. At `1` `atm` pressure, carbon dioxide does NOT exist in the liquid state, whatever the temperature.

Under exceptional circumstances, carbon dioxide can exist as a liquid: see here.