What is the little squiggly line on...

It’s called a tilde. Around the 12th century, Spanish scribes, in part to save paper, placed the tilde over a letter to indicate that it was doubled. As time passed, the mark was only used over the letter “n”; eventually, the ñ became an actual letter of the Spanish alphabet. In Portuguese, the tilde is used over vowels to indicate nasality. In other languages, it can indicate rising or falling tones. The tilde is also used in mathematics to indicate negation.