When you come across a Challenge you're interested in, you can save it for later by clicking the Bookmark icon located in the top right corner of the Challenge view. All of your bookmarks are stored in your “Bookmarks” page under "Bookmarked Challenges." Once you complete a bookmarked Challenge or no longer wish to do it, you can click the bookmark icon again to remove it from your "Bookmarked Challenges" page.
TIP: When you bookmark a Challenge with a deadline, you will receive deadline notifications for all bookmarked Challenges with deadlines. They will appear as a pip with a number on the Bookmarks icon.