When you navigate to a new page in a web browser, the page that you previously opened is usually removed from your computer's memory. There may still be cached images and other data, but if you press the return key, your browser must load most of the page again. Google Chrome developers are currently experimenting with a new cache that would make loading the previous page instant.
In simple terms, when you click on a link on a page, this feature will keep the original page frozen in memory. If you press return, the frozen page is instantly resumed. The Chrome team believes that this could "improve the performance of 19% of the navigation" on the Android version.
It should be noted that Chrome is far from the only browser to experience this concept. Firefox uses to some extent return / forward caching since Firefox 1.5 (released in 2005), but it seems to be disabled for HTTPS sites. Safari also offers a variant since 2009.
This feature is still in its infancy and there are still security and compatibility issues to be resolved. At the very least, it's an excellent use of the absurd amounts of RAM that many of Android's flagship products have.