Software Center is a self-service portal that allows you to install applications and updates on your computer. When required software updates are available for your computer a balloon notification will appear like the one below:
Clicking on the balloon notification will open an options dialog box:
When this dialog box appears you'll be presented with a couple of choices:
While applying the changes now is the recommended choice, selecting any of these options is fine. Please note, however, that if you continue to choose the Remind Me Later option without selecting one of the first two options, the updates will automatically be applied once the deadline has passed. Using the example above, if the required software updates have not been applied by the 5/19/2014 at 12:00 AM deadline, the updates would automatically begin installing at 5/19/2014 at 12:01 AM (or the next time the computer was powered on).