This is a two stage reaction.
The carbon dioxide first reacts with water to form the intermediate, carbonic acid:
CO_2 + H_2O(l) -> H_2CO_3(aq)CO2+H2O(l)→H2CO3(aq)
Then the carbonic acid reacts with sodium hydroxide to form the product, sodium bicarbonate:
H_2CO_3(aq) + NaOH(aq) ->NaHCO_3(aq) + H_2O(l)H2CO3(aq)+NaOH(aq)→NaHCO3(aq)+H2O(l)
We can combine these equations to get an overall summary of the reaction:
CO_2 + H_2O(l) + H_2CO_3(aq) + NaOH(aq) -> NaHCO_3(aq) + H_2O(l) + H_2CO_3(aq)CO2+H2O(l)+H2CO3(aq)+NaOH(aq)→NaHCO3(aq)+H2O(l)+H2CO3(aq)
This isn't very easy to read, so we cancel out H_2OH2O and H_2CO_3H2CO3 from both sides (just like you would do with a mathematical equation), and we are left with our summary equation:
NaOH(aq) + CO_2 -> NaHCO_3(aq)NaOH(aq)+CO2→NaHCO3(aq)