To yodel is to sing with a rapid, dramatic change in pitch. When you yodel, your voice veers quickly between a low register and a very high falsetto.
When someone yodels, it can sound almost like a strange cry, and you can call this sound itself a yodel as well. The yodel is often connected with the Bavarian alps, where it has a long rural tradition, although people in parts of Central Africa have long used a yodel as a form of communication, and it's been part of US country music since the 1920s. The German root word is jodeln, "to utter the syllable jo."