The first person is speaking about students in a class learning Japanese. Apparently in Japanese, the words for "rice" and "food" are very similar. The students make fun of this because it reinforces a stereotype that Asian people enjoy rice a lot, but it's pointed out that the English word "meal" can mean "the act / time of eating food" or "coarsely ground grain", making a parallel between the words for "rice" and "food" in Japanese.

The second person is saying "Humans enjoy commonly eaten foods" (being rice and grain here). They say it in a clipped way to be funny, read as "humans be like: 'staple crop'"

The third person humorously and intentionally misinterprets "staple" as a verb and pretends to assume there should be more to the second person's sentence.

The last person continues the joke, pretending that person three should staple a crop directly to their forehead. Aside from the absurdity of doing that, there's an old meme about a product called "HeadOn" that had the tagline "Apply directly to forehead" (it was a bogus homeopathic medicine that did nothing and hoped viewers would assume applying it to their forehead would have some helpful effect). The joke is further enhanced by the account being named "ublock-origin", a popular ad blocking software, since it would be strange for an ad blocking software account to be making jokes like this.