The idea behind this is that a weak acid like $\ce {CH_3CO_2H}$$\ce{CH_3CO_2H}$ will break apart, but won't break apart completely. If you have some $\ce {CH_3CO_2H}$$\ce{CH_3CO_2H}$ in water, most of it will remain as $\ce {CH_3CO_2H}$$\ce{CH_3CO_2H}$, so in the net ionic equation it is written whole, but some will separate into $\ce {CH_3CO_2^-}$$\ce{CH_3CO_2^-}$ and $\ce {H^+}$$\ce{H^+}$ ions. This relationship is quantified by the $K_a$ of the acid. Essentially, the dissociation reaction $\ce {CH_3CO_2H -> CH_3CO_2^{-} +H^+}$$\ce{CH_3CO_2H -> CH_3CO_2^{-} + H^+}$goes both ways: $\ce {CH_3CO_2H}$$\ce{CH_3CO_2H}$ decomposes into $\ce {CH_3CO_2^-}$$\ce{CH_3CO_2^-}$ and $\ce {H^+}$$\ce{H^+}$, and $\ce {CH_3CO_2^-}$$\ce{CH_3CO_2^-}$ reacts with $\ce {H^+}$$\ce{H^+}$ for form $\ce {CH_3CO_2H}$$\ce{CH_3CO_2H}$. Since the forward reaction happens at a rate proportional to the concentration of $\ce {CH_3CO_2H}$$\ce{CH_3CO_2H}$, and the backwards reaction happens at a rate proportional to the product of the concentrations of $\ce {CH_3CO_2^-}$$\ce{CH_3CO_2^-}$ and $\ce {H^+}$$\ce{H^+}$, eventually the ratio $\frac{[\ce {CH_3CO_2^-}][\ce {H^+}]}{[\ce {CH_3CO_2H}]}$$\frac{[\ce{CH_3CO_2^-}][\ce{H^+}]}{[\ce{CH_3CO_2H}]}$ becomes constant, and we call this constant the $K_a$ of the $\ce {CH_3CO_2H}$$\ce{CH_3CO_2H}$. For $\ce {CH_3CO_2H}, K_a=1.7\times 10^{-5}$$\ce{CH_3CO_2H}, K_a=1.7\times 10^{-5}$.
Now its time to connect all this back to your question. When you add $\ce{Ba(OH)_}2$$\ce{Ba(OH)_2}$ to a solution of $\ce{CH_3CO_2H}$, the $\ce{OH^-}$ ions present from the dissociation of $\ce {Ba(OH)_2}$$\ce{Ba(OH)_2}$ react with the $\ce {H^+}$$\ce{H^+}$ ions from the (partial) dissociation of $\ce {CH_3CO_2H}$$\ce{CH_3CO_2H}$. This lowers the concentration of $\ce {H^+}$$\ce{H^+}$, and so more $\ce {CH_3CO_2H}$$\ce{CH_3CO_2H}$ dissociates to keep the ratio of concentrations constant at the $K_a$. Therefore, in the presence of enough $\ce {Ba(OH)_2}$$\ce{Ba(OH)_2}$, $\ce {CH_3CO_2H}$$\ce{CH_3CO_2H}$ dissociates almost completely, even though, on its own, $\ce {CH_3CO_2H}$$\ce{CH_3CO_2H}$ dissociates only to a very small extent.