正 character is used as a tally mark in Chinese influenced cultures. It has five strokes, works exactly the same as any other tally mark you can encounter.

For anime memes, in hentai (as a genre of anime) female characters would have tally marks on them for the number on intercourses they had. As it’s predominantly produced in Japan, tally marks are represented with a 正 character, as Japanese culture is influenced a lot by the Chinese.

I should probably note that outside of the western weeb culture it’s just a regular tally mark without any lewd meaning behind it.