I know it is hard for any random person to understand "How App inventor works?". In fact, I myself have abandoned visiting this site for a month after my first visit. Because it was very hard for me to understand the bunch of options available in the design section and the blocks.
I don't want anyone to lose hope. Hence, I will remove the barrier of practising for a long time to understand the platform by describing "How it works" within 5 minutes.