Think of a function as a mini-program that works when it is triggered to start. For instance, you can have a function that tells the app to return to the previous page. But it doesn't actually take place until the user touches a Back button.
This is a function called goBack(), which tells the app to go back to the previous page:
On a page, this Back button is all set to call, or execute, the goBack() function:
<input type='button' value='Back' onclick='goBack()'>
Hope this little example helps you understand functions.