An "event" in computer science is an action or occurrence that is detected by a computer.  For example, when someone clicks the button on their mouse, it generates a “mouse click event” for the computer. If we press a key it creates a key down event.

In programming, an event handler is a part of your program that runs when specific events happen (it “handles” the event).  In MakeCode, these event handler blocks 'handle the event. The event might be on chat command 'flower' --> the player spawns a flower

There are examples on the right of 'Event ' blocks. The 'Event Handler' blocks are the things that can happen inside these Event blocks, see the examples below