When you select items in the Places panel that include time-related data, the time slider appears. This tool allows you to display or play information in a visual time sequence.
Users have created some very clever implementations that make terrific use of this feature. For example, one user created a KML-based visualization of how the London skyline has changed over the years. Once you download this file, you can select and check it in the Places panel and move the time slider so as to see what buildings were constructed when. Very cool. Read more about this on Google Earth Blog.
Do you have a favorite implementation of the time slider feature? Learn more about using the Time slider.