Why is carbon dioxide a Lewis acid?
Suppose one of the doublets of one C=O bond moves to one of these two atoms. It will not be to the Carbon atom. The Oxygen atom is a better choice. And now this Oxygen atom would be negatively charged. As a consequence the Carbon atom is positively charged and has a vacant orbital able to accept a lone pair, for example from an $\ce{OH-}$ ion or from water. This will produce the ion $\ce{HCO3^-}$ (from $\ce{OH-}$) or $\ce{H2CO3}$ (from $\ce{H2O}$).