Windows Timeline is a relatively new feature introduced with the Windows 10 April 2018 Update. It's enabled by default and keeps a chronological record of all the activities you perform on your PC – websites visited, documents edited and multimedia files consumed.
Deleting Timeline items
Sometimes, the Timeline interface (accessed within Task View from the taskbar or via the Win+Tab keyboard shortcut) might include an activity you'd rather not revisit.
To remove an individual item, open Task View and find the relevant activity in the Timeline view. Right-click its thumbnail and click "Remove" in the context menu which appears. It will be immediately deleted from Timeline.
Timeline also offers a convenience shortcut to delete all the activities recorded on a certain day. Scroll to the time period in Timeline, right-click any item displayed within the group and choose "Clear all from <time period>".
Clearing everything in your Timeline history
Deleting your Timeline history entirely – perhaps because you're also disabling the feature – is a slightly more involved process. First, open the Settings app (Win+I keyboard shortcut) and choose the "Privacy" category from the homepage.
Next, click the "Activity history" page in the navigation menu to the left of the window. Scroll down the page to the "Clear activity history" heading and press the "Clear" button. You'll need to acknowledge the prompt before Timeline removes all record of your currently logged activities.
Clearing Timeline will propagate automatically across your devices, removing all activities synchronised through your Microsoft account. You might still need to manually clear any activities on your other devices which aren't uploaded to the cloud.