Censorship in a nutshell

The hand is censoring. The plant is something good that is getting censored. The roots spreading underground as the plant is being censored shows how people just find a way around censorship by spreading similar goodness through the, as it appears here and can be applied realistically, underground.

I still don't get it

It means that if something good is censored, and people like it, it will still find a way to expand and grow.