Why is it NSFW?

No cause i'm 11 years old and i use IMGFLIP so ha ha ha. Boi

So you completely missed the meaning of this meme. Which I can't blame you cause my youngest child, who is 11 also, would have missed it too.

what is the meaning of the meme?

The identifier of the more risque memes or memes containing coarse language is labeled (N)ot (S)afe (F)or (W)ork, and are not labeled (N)ot (S)afe (F)or (S)chool. Since adults typically work while children are typically at school implies ImgFlip's target audience is adults. It even says it in their terms under the age section:
"Age
You must be at least 13 years of age to use our Services. Content on Imgflip is not targeted to people under 13 years of age."

oh I always wondered what that meant thanks.

No cause i'm 13 and I use imgflip but I act more mature than most of the adults here and use IMGFLIP so ha ha ha. Boi

That doesn't change the fact it is NSFW and not NSFS. And maybe one day you will realize you don't begin sentences with "No cause..." when having civil discussions but you do when you feel offended and are responding out of emotion. And you back up your feeling of being offended by what you deem to be an insult.

I was mocking the other commenter