The comment asking what a foot is, was building off the joke in the post image which was asking what a mile is, which itself is a reference to an American line https://knowyourmeme.com/memes/what-the-fuck-is-a-kilometer

What The Fuck Is A Kilometer, also known as WTF Is A Kilometer, refers to an American catchphrase that is satirically patriotic by being aggressively ignorant of the metric system. This is because the United States doesn't use the metric system (kilometers) and instead uses the imperial system (miles). The phrase's first usage was in a moldy meme featuring George Washington with a bald eagle and a giant firearm.

Only it started as being satire, but now it's also used by some people trying to be patriotic