Suppose you have a screen where several views depend on a single piece of data. For example, in a text editor, you can have the main editor and a small label that shows the word/character count.

Whenever the text changes, you need to update both. Normally, you would listen for user input, modify the string and update the views. Some people apparently think that this is very hard, so they decided that it would be easier that the views get "bound" to this string and update automatically (reactively): instead of you changing the string and then the views, you only change the string, and then some automatic handlers will get called that recompute the views. These triggers are called "bindings".