Pie menu is like a special menu that looks like a pie that is cut into pieces. Each piece of the pie has a different option just like a regular menu has different food items.
When you use a pie menu, you click on the center of the pie to open it up, and then you move your mouse cursor on the slice or piece of the pie that has the option you want to select. When you reach the desired slice, you click on it, and that option will be selected.
It's called a pie menu because it looks like a pie that you would eat at a party, and each slice of the pie has a different option, just like each slice of a real pie has a different filling. Pie menus are often used in computer programs to make it easier and faster to select a particular option or tool.