Google Timeline promises you that you can 'revisit the world that you have explored'. Critics may well say that Google is keeping very close tabs on everything that you do, even moreso than you thought! They explain: "Your Timeline allows you to visualize your real-world routines, easily see the trips you’ve taken and get a glimpse of the places where you spend your time. And if you use Google Photos, we’ll show the photos you took when viewing a specific day, to help resurface your memories."
Let me give you an example. Last year I went on holiday to Cornwall. I used my mobile phone a few times to check mail, take photographs (though at that point not on Google photos) and check the web.
So I can see where I went, how long I stayed, and if I'd used Google photographs I could see what photographs I had taken. I can go through my history to any given year or date to see what I did and where I went, I can see popular locations, favourite restaurants and so on.
It's currently available on the web (you can see yours at https://www.google.com/maps/timeline) if you have location services turned on, and it's also on the Android at the moment, not iOS. It's being rolled out, so don't worry if you don't have it yet, it'll come your way soon. You can of course turn it off by choosing not to share locations with Google, and you can delete particular days or journeys for whatever reason. Obviously you're the only person who can see this; it's NOT public.
I'm really in two minds over this. It's a great tool, and if you ever need to remember where you were on a particular day, or want to relive memories of a holiday, it's superb. However, it's also worrying that we're continually feeding huge amounts of data into Google, and they can do pretty much whatever they want to with it.