The word you type might be a simple definition such as + which replaces the top two numbers on the stack with their sum. So for example if you had typed 5 and then 6 the stack display will show ...5 6 and if you then type + the stack display will now show ...11
The word may do a great deal more. It might compile and run an extensive utility or application, which might put up a customized graphic or text screen, and might give you a completely different keyboard interaction while it is running. The colorForth editor is such a program.
When any colorForth word or program you have invoked from the keyboard completes, control always returns to the keyboard interpreter.