Unix Time, or a timestamp, is a method of tracking time by determining the approximate number of seconds from a particular event. That event is called an Epoch. Since this time format is only off by a few seconds each century, it is usually considered good enough for most applications.
Unix time is (currently) a ten digit number, and looks like this: 1229362315. WordPress often uses a Unix timestamp internally to track time. The human readable times and dates you see are converted from Unix Time or from a MySQL DATETIME field.