You can bookmark an answer, post, or share by clicking the 'more options' menu (e.g. the "...") at the bottom of the content, then selecting the "Bookmark" option.
On web, you can find your Bookmarks by clicking on your profile icon in the top right-hand corner, then selecting "Bookmarks" from the drop-down menu. You can also get to this page by visiting: https://www.quora.com/bookmarks.
On the mobile app, you can find your Bookmarks by tapping on your profile icon in the top left-hand corner of the app, then selecting "Bookmarks" from the drop-down menu.