Think of it like an itch you can't scratch - the urge to get up and go somewhere else is so strong, it feels like a physical discomfort.
Here are some ways the idiom can be used:
* "After a year in the same job, I started to get itchy feet and decided to look for something new."
* "She's got itchy feet again and wants to go backpacking around Europe."
* "He's always had itchy feet and has lived in five different countries in the past ten years."