Timeline lives in the Windows app switcher. When you click it, you'll see your active apps, but below that you'll see what you were running earlier in the day. Clicking down on one of those things that you were using earlier will pop it open just as you were using it before. This works across multiple devices, as well. When you open up another Windows device where you're signed in, you can resume the tasks you were using before. This will even work across other devices like an iPhone using the Cortana app. If you're somewhere where you have Cortana, it prompts you to continue working on whatever you were doing before. If you don't have the specific app installed on your phone, it points you to the right app.
At first glance, it sounds a little bit like the Time Machine backup feature that Apple has included in macOS for years now. But Time Machine is more of a file backup system that lets you go back and see earlier versions of files that you might want to restore. Microsoft's Timeline covers applications and websites as well as just files, and it doesn't require an external hard drive, as it's not really a true backup system in the way Time Machine is. Indeed, Timeline appears more like Microsoft's answer to Continuity, a feature Apple build into macOS and iOS that lets you pick up and resume work across whatever Apple device you're using.
Timeline is just one feature in the forthcoming Creators Update, which features a host of tools for using Microsoft's software and services across devices. The "Microsoft Graph" set of APIs will let you pick up and continue work across multiple devices and will iOS and Android as well as Windows. It also gives you have a "universal clipboard" across your devices.Click here to catch up on the latest news from Microsoft Build 2017.