Timeline is an application that allows you to sign in with your Google email account onto a device that you use or travel around with, the device that the email is signed in on has to have the location on so that the google history timeline can access where you are and where you've been hence the name of the app is "Where you've been".

Signing in and using Timeline is very simple, all you need to do is to install the application on the device you want to use or travel around with, open the application, sign in with your Google account, and you're done.