What did he mean by that?

He has neglected his parasol

>>1069748
It's actually a very complex metaphysical criticism of religion in context of social standarts during the renaissance.
By forgetting his umbrella he let's the sun (=god) shine upon his head which prevents him from rationally evaluating the world and reach through enlightement.
The real genius doesn't get shined on, he shines from within.

>>1069781
*true

>>1069781
Also the forgetfulness part clearly refers to Plato's epistemology and the acquisition of knowledge as remembrance. So what he's saying is that by discarding the mediation of the forms he can approach truth directly with his mind (head). There is also an element of risk since a rain may conceal the sky entirely and even make him catch a cold, but this is the price of immediate knowledge.

He means that he failed to remember to bring with him his metal pole with a canvas shield at the end designed to keep rain off of himself.