You don't have to understand memes...
This is a roll safe meme. This breaks the standard format of the meme (you can't x if you don't y), but still fits with the spirit of the meme fairly well.
The reason why this is funny is that it is encouraging people that fail to understand memes to insinuate that they understood the meme by pointing at their forehead in a knowing way. It is sort of like the memester was assuming that the person that is reading the meme is too stupid to understand memes, and is offering them helpful advice.
I find this pretty funny. It is like the opposite of my blog - I try and help people understand things like this, and he is just saying: "Pretend you understand it"
Either way, it is insulting to the reader because it presupposes that the reader cannot understand memes.