It's not available on every single Street View image, but for the ones where it's enabled, you click the small clock icon on the top left of the screen.
The archive goes back to 2007, the year that Street View first began. While that might not seem like ancient history right now, over time it'll mean that there's a record of what our streets looked like every year into the future. Which is pretty cool.
That's essentially how Google suggests people use this new feature - to watch incremental change happen quickly. For instance, you can watch New York's
Freedom Tower One WTC being built:
Or witness the devastation wrought by Japan's 2011 Earthquake:
For me though, the fun of Street View has always been in using it as a floating nostalgia camera. I like virtually walking by the first apartment I lived in, or standing at the edge of the driveway I grew up in. Sometimes, the Street View cameras will catch some glimpse of your actual life, like a locked up bike or a recognizable open window. It's nice to think that those things will be preserved, rather than rolled over as the images are updated.