変顔, or hengao translates into something like "weird face" or "strange face". While Japan is usually about the cute and clean image, hengao is all about making the oddest face imaginable. While she isn't the first to do so, the model and singing idol Kyary Pamyu Pamyu has popularized the term.
In my opinion, she's the best at it. Even though it's about looking strange, some of them are pretty cute.