The button 
interrupts evaluation in the Listener. The break gesture key stroke Alt+Ctrl+C (and the Interrupt Lisp button, in the GC Monitor window in the Motif IDE only) can also be used.
This is useful for stopping execution in the middle of a loop, or for debugging. When the interrupt is processed, the debugger is entered, with a continue restart available.
If the Listener is not evaluating a form, then the Process Browser will be opened allowing you to interrupt a background process.
LispWorks IDE User Guide (Unix version) - 01 Dec 2021 19:37:17