When you navigate to a new page in a web browser, the previous page you had open is usually discarded from your computer's memory. There might be cached images and other data left over, but if you press back, your browser has to load most of the page again. Google Chrome's developers are experimenting with a new 'back/forward cache' that would make loading the previous page instant.
Simply put, when you click on 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 this could "improve performance up to 19% of all navigation" on the Android version.
It's worth noting that Chrome is far from the only browser to experiment with this concept. Firefox has used back/forward caching to some extent since Firefox 1.5 (released in 2005), but it seems to be disabled for HTTPS sites. Safari has also had a variation of it since 2009.
This feature is still in the early stages, and there are still security and compatibly issues to iron out. At the very least, it's a great use for the absurd amounts of RAM that many Android flagships now have.
- Google Developers