Solubility of Aluminium Hydroxide in Sodium Hydroxide Solution
That's because $\ce{Al(OH)3}$ is amphoteric in nature, it acts as both acid and base. When you drop a chunk of $\ce{Al(OH)3}$ into a solution of $\ce{NaOH}$, you would get a soluble salt, which is sodium aluminate, and some extra water.

$$\ce{Al(OH)3 + NaOH -&gt; Na[Al(OH)4]}$$