It's passing to this function called toupper, which if you've never used it it actually does what it says it makes it touppercase.

它把这个传递给toupper函数，如果你们使用过，它表示的是把小写字母,转换为大写字母。

哈佛公开课 - 计算机科学课程节选

A If you want to convert a number like 65 to the letter A, you just have tell the computer "cast that int to a char."

如果你要把一个像65这样的数字转换为字母，你只需要告诉计算机“把那个int型数据转换成char型处理“

哈佛公开课 - 计算机科学课程节选