What is the tip-of-the-tongue phenomenon?

Isn’t this that who? It’s that who. Ah! Who’s that? Why can’t I remember? This song is so familiar, my mom loves it… right, what’s it called? I’ve memorized the words last night, but this morning I can’t remember them, I know where they are in the word book, but I can’t remember how to write them! Help! What the hell is going on here?