When you navigate to a new page in a Web browser, the previous page that you opened is normally discarded from your computer's memory. There may be cached and other data remaining, but if you press again, the browser will need to load most of the page again. Google Chrome developers are experimenting with a new "back / forward cache" that would load instantaneously from the previous page.
Simply put, when you click a link on a page, this feature will keep the original page frozen in memory. If you press back, the frozen page is instantly resumed. The Chrome team estimates that this could "improve performance by up to 19% of all navigation" in the Android version.
It's worth noting that Chrome is far from the only browser to experience this concept. Firefox has used the back / forward cache to some extent since Firefox 1.5 (released in 2005), but seems to be disabled for HTTPS sites. Safari has also had a variation since 2009.
This feature is still in the early stages, and there are still security and compatibility issues to solve. At the very least, it's a great use for the absurd amounts of RAM that many Android flagships now have.