The IBM5250 emulation locks the keyboard on transmit (for example on Enter or Function keys) and Busy is displayed on the status bar until the host responds and unlocks the keyboard. If the host response is slow you may also see a heart beat icon in the status bar.

If the busy is not cleared, it means the host didn't respond.

The busy status by pressing Ctrl+B, this will allow the user to attempt the resend or correct an invalid key press not handled by the host.