A signal is emitted when something happens. For example, a window is created, a window is destroyed and so on. The signal “activate” is emitted when the application is activated. (Activated is a bit different -from started, but you can think the both are almost same so far.) If the +from started, but you can think of them both as almost the same so far.) If the signal is connected to a function, which is called a signal handler or simply handler, then the function is invoked when the signal emits.
The flow is like this: