Handlers are a key element of HyperCard programming. A handler is a command. Handlers come in pairs. Handlers must begin and end all active HyperCard script. When a button has been created the default handlers ON MouseUp and End MouseUp are automatically created.
Handlers can be any keyboard command or mouse command.
MouseUp is activated when the mouse has been depressed and let up. User created handlers are not based on actual computer events. The CallMe handler might be written in the background of a stack and script might be entered after it. End CallMe would end this user created handler. HyperCard has in it error protection for incorrectly written code.