Operational Calculus: $\left ( 1-D \right )^{-1} x^3$ expanded as the geometric series $\left ( 1+D+D^2+D^3+\cdot \cdot \cdot \right )x^3$

Question

While watching a YouTube video by Supware, titled "The Abstract World of Operational Calculus", the speaker is working with the derivative operator $D$. He says he will "formally" expand $(1-D)^{-1} x^3$ as a geometric series, and proceeds to replace it with $$\left ( 1+D+D^2+D^3+\cdot \cdot \cdot \right )x^3$$

Could someone please explain how he did it? I have no expertise in this area at all, so I'm not sure how he did that.