Kernel Input Dialog Box
This dialog box is displayed when the kernel commands Input or InputString are evaluated in a notebook.
Any text you type into the dialog box is sent to the kernel for evaluation, when you click OK. The result of the evaluation is returned as the value of the Input or InputStringexpression.
This dialog box provides an alternative way to supply input to the kernel. This is useful, for example, in writing programs that prompt the user for input.
See Reference Guide: Input, InputString.
See The Mathematica Book: Section 2.9.20.