Neko Funjatta, or I Stepped on the Cat, is the Japanese name for a simplistic 2/4 time piano tune. It's composer is unknown, and the piece goes by several different names depending which language you speak.
"I Stepped on the Cat" is the only piano piece that Itsumi Momose knows how to play. Ren knew this so he used her inability to play any other piano pieces as a plot device for the improvisation he was doing. Itsumi figured his motive by figuring out her character's emotion and finding out that he purposely asked her to play the piano despite her inability because he knew her character would hesitate as well.
After that, Ren as Katsuki asks Itsumi for them to just play together, which shocked everyone in the set. They thought he was going to play Chopin's "Fantaisie" like the original actor who played Katsuki, which is a difficult piece to learn and is the main theme of Dark Moon.
But instead, Ren started to play something different. It was a poor version of "I Stepped on the Cat" in an octave lower. After that, he invited Itsumi to play it with him and with them playing it together, Ren actually memorized her hand movement and the two of them were able to play a happy piece of "I Stepped on the Cat" and finished the scene.