Traditionally, Arduino programs are written in a sequential manner, which means that whenever an Arduino program needs to synchronize with some external event, such as a button press, arrival of a character through the serial port, or a time delay, it explicitly waits in-line for the occurrence of the event. Waiting "in-line" means that the Arduino processor spends all of its cycles constantly checking for some condition in a tight loop (called the polling loop).
Although this approach is functional in many situations, it doesn't work very well when there are multiple possible sources of events whose arrival times and order you cannot predict and where it is important to handle the events in a timely manner. The fundamental problem is that while a sequential program is waiting for one kind of event (e.g., a button press), it is not doing any other work and is not responsive to other events (e.g., characters from the serial port).
For these and other reasons experienced programmers turn to the long-know design strategy called event-driven programming, which requires a distinctly different way of thinking than conventional sequential programs...
The following YouTube video shows how to use the graphical QM Modeling Tool to replace the standard Arduino IDE and how to use it to program Arduino graphically.