An alert box is a message that appears on the screen to warn you of some imminent disaster, or to inform you of a situation in which your actions may have irreversible consequences.
For example, if you've just chosen the command to delete a file, an alert box might warn you that the file will be gone forever, and ask you if you really want to delete it. You have to deal with the alert box before you can move on with your life, meaning your only option when you see something like this is to click one of the buttons it offers you. Typically the dark-bordered button (which is the default button) is the safest option available-and because it's the default, you can hit the Return or Enter key instead of clicking the button. If the alert is telling you the computer crashed, then the button may not even work, in which case you just have to turn off your machine by its switch or, if you have one, press your reset switch