What does Piccolo telling Gohan to "dodge" mean? I have been seeing some posts on Facebook recently with Piccolo yelling at Gohan to dodge. I do not understand the reference behind it. 

What incident is being referred to?
This is a reference Team Four Star's DragonBall Z Abridged series.

In it, Picollo is teaching Gohan to dodge. However, he ends up conditioned to freeze in fear of the word instead. This becomes one of the most popular running gags in the series.