$$\text{RMgX}+{\text{CO}}_2\stackrel{\text{dry ether}}{\to }Y\stackrel{{\text{H}}_3{\text{O}}^{+}}{\to }\text{RCOOH}$$
What is $Y$ in the above reaction?

The nucleophilic Grignard reagent (${\text{R}}^{\delta }-{\text{Mg}}^{\delta +}\text{X}$) attacks the carbon of ${\text{CO}}_2$.
$$\text{R}-\text{MgX}+\text{O}=\text{C}=\text{O}\to \text{R}-\text{C}(=\text{O})-\text{O}-\text{MgX}$$
The intermediate $Y$ is the magnesium salt of the carboxylic acid: ${\text{RCOO}}^{-}{\text{Mg}}^{+}\text{X}$.